Jump to content
Sign in to follow this  
cozye

78 280z, slight stumble, low vacuum

    Recommended Posts

    I've got a 25 page thread going on another forum and have been pulling my hair out for about a month on this car. Thought I'd post over hear and maybe get some fresh ideas.

    History.

    I bought this car about a month ago. I'm the 3rd owner. Car has 108k miles on it. Solid body, garage kept for it's entire life exept the last two years. Original owner was a female who bought the car out of med school in 78 and garage kept it for the first 95,000. Records in the car show that the original owner had 89000 miles on it in the mind 90's, so it hasn't been driven much at all in the last 15 years. Original owner supposedly only drove it a few times each year and would get the oil changed.

    2nd owner didn't do much maintenance to the car and had it for about 13k miles. It was running very rough when I picked it up, exhaust manifold gasket leaking, plugs worn out, bad distributor cap, slipping clutch, etc.. 2nd owner said when they picked up the car it ran pretty good, but had a slight stumble/hesitation at lower rpm's.

    This is what I've done.

    Replaced all tune up parts, ngk plugs, cap rotor, wires, etc..

    new exhaust/intake manifold gasket. no longer leaks.

    new clutch/pressure plate, etc..

    fuel pressure test - shows 37psi with FPR disconnected. 33psi at idle

    vacuum at idle is currently 15.5hg

    carb sprayed all areas of intake, head, vacuum connections, throttle body, etc.. couldn't find any leaks

    rigged up a smoke machine to intake, smoke tested it 3 or 4 times, couldn't find any leaks

    rigged up a "boost test" to try and find vacuum leak, around 2 or 3 psi with air compressor connected to brake booster vacuum connection. Couldn't find any leaks

    compression test when I first got the car showed all cylinders getting between 145-155 psi. Might be a little better now after valve timing and adjustment and oil change (old oil was thin and gas)

    valve timing check showed still in spec, but went on and put gear at 2nd position which got the timing mark perfect like new. Valves adjusted, but were very close (only a few off by .001)

    been through the FSM and performed full check on entire EFI system, everything checks out ok. I've done this about 3 times, including air bypass, thermotime, etc..

    Pulled AFM off car and performed full resistance test, and voltage output test with 12v applied.

    Replaced ignition with crane xr700 optical setup. Cleaned and tested vacuu advance mechanism. Vacuum advance dashpot holds vacuum and doesn't leak.

    EGR valve replaced

    Water temp sensor tested ok, replaced it anyway since it was cheap.

    All injectors were pulled off, sent away for cleaning and flow testing.

    Vacuum on the car was running around 14hg, I did a sea foam treatment and got it to 15.5hg.

    The car has a stumble, especially noticeable at lower rpm. Runs a bit lean. Can't figure out if it's lean due to wrong A/F mixture, or motor pulling low vacuum for some unknown reason. If I add a bit of fuel buy faking the system (moving AFM gear, or jumping TPS to full enrichment), it smooths out quite a bit. It does it worse when it's warmed up and hot.

    I've done quite a bit of testing, plugging vacuum lines, etc.. and can't pin it down. I've had quite a bit of suggestions from another forum and have tried a lot of things, maybe some things I've forgotten to list above.

    Any help would be appreciated.

    Share this post


    Link to post
    Share on other sites

    That's a lot of detail, except for the actual problem description. Is there any way you could get a video of the stumble? Does it fall flat when you hit the gas, or does it start coughing, or does it pop and buck, hesitate and go...? Does it do it with no load, or only in gear? 1st, 2nd, third or all?

    And "lower rpm" could mean 800 to 1000 or 1500 to 2000. Are you talking about just off idle or as you accelerate or cruising at low rpm?

    You didn't mention your timing either.

    Just trying to help, it's hard to tell what the problem is.

    Share this post


    Link to post
    Share on other sites

    I've tried timing at 10-15 degrees, doesn't make a huge difference where I time, except that the more advanced timing will lessen it slightly, but too much advance causes it to accelerate slightly less at higher rpm. I've got it set on about 11 now, 10 is what the manual calls for.

    It's running a bit lean, at worse you will get the accasional intake pop. It seems to vary depending on outside temp a little, and how warm the car is. The warmer the car is the more it stumbles, but it's not really a huge difference. It still does it when it's cold too.

    So when taking off in 1st gear it's the most noticable, mostly I believe because you are at lower rpm and any hesitation or stumble in first is going to be felt a lot more due to the gearing. It will do it in all gears, but the higher the gear and the higher the rpm the less it is felt. I would say about 2500 rpm and up you kind of have to be paying attention to notice it.

    Also, it's worse at slight or partial throttle openings. the TPS switch has been tested thoroughly, but when you give the car a lot of gas (30-50% throttle or more) it's either unoticeable or non existant.

    It has a slight stumble at idle, almost like the timing is wrong or it's got a hot cam in it (it doesn't, it's 100% stock), it will do it worse under a load.

    Here is a video taken a couple weeks ago. I've got the vacuum up a little more than this now but it's doing the same thing. The injectors aren't as loud since I've had them cleaned up, if you get to the part of the video where I'm by the exaust, that's the stumble you hear it just kind of comes across a bit high pitch from the video camera

    http://www.youtube.com/watch?v=RGj3CQ_SCks

    Share this post


    Link to post
    Share on other sites

    The video and description help a lot. I don't have any ideas right now but did notice two things that seem unusual, compared to the two engines that I have played with the most, my 76 driver and a 78 parts car.

    The ticking noise seems much too loud to be injectors, it sounds more like a rocker arm or valve noise. My two engines are nowhere near that noisy, I've adjusted the valve lash on both. And your dwell/tach/volts meter showed 600 rpm, but I'm sure that your engine was up around 1200 to 1500 rpm (what did the dash tach show?). Maybe you had the setting on 4 cylinder instead of 6?

    The noise from the exhaust would be what I would call a "miss". Just for future reference. If you could drop the idle down (way down, to get a bigger effect), then pull the injector plugs off one by one (some people pull the plug wires but the stray sparks can be problematic), you might find the cylinder that is the source of your miss. The rpm should drop about the same amount for each cylinder that is working correctly. RPM won't drop as much for the weak cylinder. I think it is called a"power balance" test by some.

    Share this post


    Link to post
    Share on other sites

    The injector noise isn't as loud now that I got them cleaned and flow tested. It was two injectors that were making the noise. Too fast for valve noise as you could pull the oil cap off and watch the cam lobe, which wasn't turning near that fast. You could put a screw driver on the injectors with your ear up to it, it was obvious.

    I've run through the valve adjustment twice, once when I first got it and again just last night to double check.

    The tach reading was right, it doesn't have 4/6/8 setting, just different scales. You were probably looking at the scale on the top and not the one on the bottom. It was at 800 rpm.

    The reason I haven't called it a miss is that it doesn't have a rythem, and is sporadic. I think it's actually just slight lean popping. The more gas you add to the A/F mixture, the more it goes away. When it does it badly, it will "afterfire" back through the intake.

    I've done the balance test, bunches of plug reads, etc.. All cylinders are firing.

    Share this post


    Link to post
    Share on other sites

    Like I said, your idle sounded like 1200 - 1500 in the video. Sure didn't sound like 800. Doesn't matter I guess. If that's an old video, it might help to make a new "state-of-the problem" video.

    The point of the balance test isn't "all cylinders are firing" but "all cylinders are producing equal power". Or not. So you have to measure the rpm drop. For example, if you have a cylinder that drops spark every third firing, you might be able to measure its impact.

    I think that you might be at the point where you've done almost everything and decided that what you've looked at is all good. And you're looking for something that you haven't heard before. It's tough to help the guy that has done that and done that, etc.

    The miss from your exhaust sounded like a typical misfiring spark plug, for what that's worth. Have you done the "look for sparks in the dark" test?

    Here is one possibility for the low vacuum reading that might have merit - have you checked the PCV hose itself (the one underneath that connects to the valve) for holes or cracks or leaking junctions? I think that your pressure test of the intake system would miss that, because it would cause the valve to close, blocking off the crankcase. And the other parts of the system run through ported vacuum. Actually, I think that your pressure test only tested the manifold side and not the crankcase portion, so that might be worth double checking. The throttle valve closed off one end and the PCV valve the other.

    One other way to check for vacuum leaks (although without actually finding it) is to close the idle screw down all the way. The motor should die. If it doesn't, air is getting past the throttle body or leaking in from outside. I did that on my 78 motor and found that the Air Regulator was stuck open (probably not your problem, just an example).

    Edited by Zed Head

    Share this post


    Link to post
    Share on other sites

    External tach and dash tach agree on 800 rpm. The video was taken on an iphone, I think it has the tendancy to magnify high frequency sounds and not the lower frequency, so it makes it sound faster and makes the injector ticking seem louder than it really is.

    Yeah, i've looked for the spark in the dark test.

    PVC valve is new, PVC valve hose is new. Oil dipstick, oil filler cap, etc.. checked. valve cover gasket is new. rear main seal is new.

    car dies with idle screw turned all the way in.

    timing light put on all cylinders too and watched reference point on crank for irregular pattern or skipping trigger on light. I've got a brand new ignition system on the car. It's firing flawlessly.

    I'll try the balance test again with a much lower rpm and see if I notice anything unusual.

    Share this post


    Link to post
    Share on other sites

    balance test at 500 rpm didn't reveal anything. would drop equally with each cylinder pulled.

    Share this post


    Link to post
    Share on other sites

    I had a BMW Z3 that had sat up a lot before I bought it. It missed lightly at idle just like your engine, although vacuum was normal. I did a couple of flushes with Gunk to dissolve out the varnish, and that freed up the valves. It purred like a kitten thereafter.

    I'm currently working on similar issues on my own '78 Z. I'll let you know if I discover anything, but my approach is a shotgun approach at this moment. I've disassembled and checked the entire fuel/intake system and manifolds, and I'm currently reassembling. I had some obvious intake leaks, and my injectors were very worn. I'll know very soon whether new injectors, tight manifolds, cleaning and lubing will have solved my engine's problems.

    Share this post


    Link to post
    Share on other sites
    I had a BMW Z3 that had sat up a lot before I bought it. It missed lightly at idle just like your engine, although vacuum was normal. I did a couple of flushes with Gunk to dissolve out the varnish, and that freed up the valves. It purred like a kitten thereafter.

    I'm currently working on similar issues on my own '78 Z. I'll let you know if I discover anything, but my approach is a shotgun approach at this moment. I've disassembled and checked the entire fuel/intake system and manifolds, and I'm currently reassembling. I had some obvious intake leaks, and my injectors were very worn. I'll know very soon whether new injectors, tight manifolds, cleaning and lubing will have solved my engine's problems.

    Yeah, let me know how it goes. I've already dissassembled my intake fuel system, new intake gasket and inspected AFM throttle body, new injector gaskets, etc.. and put it back together. Didn't help me.

    Your M3 story is good info though. As I mentioned before, my first sea foam treatment brought the vacuum up 1.5. It may be just gunk, goo, and carbon build up on the top end causing my issues. I did advance my AFM module spring 5 teeth to get it driveable and it is pretty drivable with that setting although the slight stumble is still there.

    I think I'm about at the point of just cleaning/lubing the top end a few times with a seafoam or similar, and just getting some miles on it and see how it does.

    Share this post


    Link to post
    Share on other sites

    Cozye, I'm more or less finished with my engine. It runs great now. Check out my "Purs like a kitten" thread.

    The final step that got my engine running normally (bringing up the vacuum, smoothing all the misses at idle, stopping the light backfiring issue) was my adjustment of the AFM. I had to enrichen the mixture by TWELVE TEETH! After I was finished, I had a vacuum of 19 in Hg, with no misses and a much less labored exhaust sound. When I was finished, the engine RPM dropped roughly equally with a push of the AFM vane in either the open or closed position.

    I'll add that I was doubtful about needing to enrich the mixture, because I could smell unburnt hydrocarbons in the exhaust. However, I think the unburnt hydrocarbons were from the misses. I'm guessing that a cylinder would occasionally miss because of too lean a mixture, and the unburnt fuel would enter the exhaust. Then it would ignite, burning inefficiently, creating a hard puff. I would smell what was left of the poorly burnt fuel. Even though I kept enriching the mixture, I didn't really get fumier exhaust. If anything, it smelled cleaner as the mixture got richer. Go figure.

    Anyway, the AFM adjustment seemed to be the magic juice for my engine. Maybe it will work for your engine too.

    Good luck!

    Edited by FastWoman

    Share this post


    Link to post
    Share on other sites
    Cozye, I'm more or less finished with my engine. It runs great now. Check out my "Purs like a kitten" thread.

    The final step that got my engine running normally (bringing up the vacuum, smoothing all the misses at idle, stopping the light backfiring issue) was my adjustment of the AFM. I had to enrichen the mixture by TWELVE TEETH! After I was finished, I had a vacuum of 19 in Hg, with no misses and a much less labored exhaust sound. When I was finished, the engine RPM dropped roughly equally with a push of the AFM vane in either the open or closed position.

    I'll add that I was doubtful about needing to enrich the mixture, because I could smell unburnt hydrocarbons in the exhaust. However, I think the unburnt hydrocarbons were from the misses. I'm guessing that a cylinder would occasionally miss because of too lean a mixture, and the unburnt fuel would enter the exhaust. Then it would ignite, burning inefficiently, creating a hard puff. I would smell what was left of the poorly burnt fuel. Even though I kept enriching the mixture, I didn't really get fumier exhaust. If anything, it smelled cleaner as the mixture got richer. Go figure.

    Anyway, the AFM adjustment seemed to be the magic juice for my engine. Maybe it will work for your engine too.

    Good luck!

    Funny that you arrived at 12 teeth. That's exactly the number I arrived at when richening mine up that way with the AFM . I also got up to about 18 or so vacuum when doing that.

    However, I think that when you go drive the car you may find that it's too rich at mid to full throttle. This is the reason why I put mine back, trying to find another option. I think richening it up with the AFM might be compensating for some other issue. I could not get over this idea. The cure might definitely be a richer mixture at lower rpm, but the upper rpm gets richer too with the AFM. Let me know what you think when you drive it.

    Share this post


    Link to post
    Share on other sites

    The other way to richen the mixture at idle but with little effect anywhere else is by the idle air bypass screw (it might be called the lean adjustment screw or something similar) on the AFM. Turn it in to richen, then adjust the idle speed with the throttle body screw.

    It's behind the rubber plug on the bottom, side of the AFM. I have a 76 and I had to adjust mine to keep the flapper off of the fuel pump contacts.

    Haven't been emission tested since then so I don't know where it will come in.

    Share this post


    Link to post
    Share on other sites

    The other way to richen the mixture at idle but with little effect anywhere else is by the idle air bypass screw (it might be called the lean adjustment screw or something similar) on the AFM. Turn it in to richen, then adjust the idle speed with the throttle body screw.

    It's behind the rubber plug on the bottom, side of the AFM. I have a 76 and I had to adjust mine to keep the flapper off of the fuel pump contacts.

    Haven't been emission tested since then so I don't know where it will come in.

    Yeah, I thought about that too, but the lean condition actually exist at idle and at slight throttle openings such as cruising or light acceleration. Thats when you get the intake pops, etc..

    The best way to richen it up I found was to jumper the "full enrichment" position on the throttle position switch, but you use a lot of gas doing that ( i got like 13 mpg), and it's a hack. I'd rather figure out the real problem then just put in a hack.

    Where I've got it now makes it plenty driveable and is a work around for now, I've got the AFM advanced 5 teeth, and the timing on 15 degrees advance. That almost completely gets rid of the stumble, appears to get decent mileage, and doesn't loose power so much. It's a compromise for now.

    it seems to me that there are a few 78 owners that have this issue. I know of 3 so far.

    Share this post


    Link to post
    Share on other sites

    Behind the rubber plug? There's a rubber plug?

    Hmmmm... Not on mine. Just the screw, which is roughly flush with the housing (i.e. so there would be no room for a plug). Should it be screwed in quite a bit more than that? Perhaps that was an adjustment the PO made? (I was hoping that adjustment went unmolested.)

    Grrrrrr....

    Well, I'll do the test drive tomorrow. I'm guessing I'll be too rich at half/full throttle and will indeed need to wind up the AFM again -- and advance the idle mix screw quite a lot to richen the mix at idle. Anyway, one thing is for certain: Given the right fuel/air mix, the engine runs fine. (Same in your case, Cozye.)

    Share this post


    Link to post
    Share on other sites
    Behind the rubber plug? There's a rubber plug?

    The rubber plug is actually pretty deep, about 3/8 to 1/2" I would guess. And mine fits flush so the screw must be in pretty far. I can't see the head of the screw. But my AFM is a rebuilt one from MSA (or wherever they source theirs from) so I don't know if it was ever properly adjusted. Apparently the stock AFMs are factory-adjusted. But I did end up screwing mine in even farther and the engine idles fine, and accelerates smoothly.

    I have rethought my original comment about no effect anywhere but idle and I think now that there must be some effect just off idle until enough air is flowing to offset it.

    One more thing to play with...

    Edited by Zed Head

    Share this post


    Link to post
    Share on other sites

    Bit off topic here. I've had this same issue earlier when I bought my Z. The previous owner decided that rich was just not rich enough, and had it set so rich that it barely idled at anything lower than a 1k. When I went to pass emissions I failed with CO levels in the 2k range.

    Secondly, that ticking noise is the injectors. I have the same noise (it drives me nuts) when the car gets warm after driving on the freeway. I've cleaned and cleaned them with everything under the sun (other than take them off) and nothing seems to help.

    Jan

    Share this post


    Link to post
    Share on other sites

    Secondly, that ticking noise is the injectors. I have the same noise (it drives me nuts) when the car gets warm after driving on the freeway. I've cleaned and cleaned them with everything under the sun (other than take them off) and nothing seems to help.

    Jan

    You might consider having them cleaned off the car professionally. Mine still tick a little, but I cannot hear it in the car now and it did seem to help.

    Share this post


    Link to post
    Share on other sites

    Well, I'll do the test drive tomorrow. I'm guessing I'll be too rich at half/full throttle and will indeed need to wind up the AFM again -- and advance the idle mix screw quite a lot to richen the mix at idle. Anyway, one thing is for certain: Given the right fuel/air mix, the engine runs fine. (Same in your case, Cozye.)

    Definitely let me know what you think. I think we are fighting the same problem, and two heads (with two cars!) are better than one.

    Share this post


    Link to post
    Share on other sites

    I know this is a old thread but I have a idea.

    Your fuel pressure is a hair low, volume maybe too?

    Easy test is unhook fuel line after filter and run pump for 15 seconds in to a plastic QUART milk jug. Should fill it half way or more. I did this test and only got 25% fill. Half of what it should of been.

    I was having same problem on my 75 z. Just got the car and it had sat for a long time. Well it had a new airtex fuel pump on it and the guy I bought it from said the tank had been cleaned 3 months earlier. Read bad reviews about airtex.

    I knew it was running lean because of the intake pop. So I bought a MSD fuel pump to install.

    Well when I took off the airtex guess what I found ?? a chunk of rust caught in the fuel pump nozzle. Obviously they changed the fuel pump BEFORE cleaning the tank!! New pump and rust dug out helped a lot. But as soon as it cools off enough I intend to remove and clean and reline the fuel tank. At the same time I will remove and clean fuel dampener and blow the lines while dampener is off (it acts as a flow stopper to tank).

    I too cleaned every thing from fuel filter to injectors and thought I was safe, but on 35 year old cars, got to think fuel tank to injectors. Rust catches in the darnest places.

    just a thought ...hope it helps

    Great tip about cranking idle screw down to see if it dies, checking for vacuum leak.

    love my Z

    Rich

    Share this post


    Link to post
    Share on other sites

    I guess Im a tad bit late to this party.

    Cozye, I believe I have the same issue as you. I have pulling out my hair trying to figure it out. Im about to replace the injectors then try the fuel pump (which has been replaced but does look real old). Did you ever figure out the issue?

    Thanks!

    Share this post


    Link to post
    Share on other sites

    Would I be out of line to suggest it's time to hire a professional mechanic and get it fixed instead of jerking around on the Internet?

    Share this post


    Link to post
    Share on other sites

    Create an account or sign in to comment

    You need to be a member in order to leave a comment

    Create an account

    Sign up for a new account in our community. It's easy!

    Register a new account

    Sign in

    Already have an account? Sign in here.

    Sign In Now
    Sign in to follow this  

    ×
    ×
    • Create New...

    Important Information

    By using this site, you agree to our Terms of Use. We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.