Tachometer relationship with TPS

[johnny haywire]

Just a quick question for you FI gurus. I've got a few issues with the FI on my '78 280z (manual). She chugs a bit starting up when it's over 70 degrees F, then settles into a high idle, then comes down to what I believe is about 750 RPM, but this can take a while. She runs pretty well after being warmed up, no sputtering or front/back fires, but runs very rich. The exhaust is always very fumy and the fumes get into the car with the right type of breeze.

But before I go about testing different FI bits, I was wondering, since my tach currently doesn't work, does the TPS (or ECU) count on a tach signal for its operation?

I'm hoping that getting my tach working or replaced might save me some digging around the FI system.

Many thanks in advance!

[Wade Nelson]

The throttle position sensor works in combination with the tach signal.

For instance, if you are at 3000 rpm, engine braking down a hill, and let completely off the gas, the TPS senses "idle position) and the ECM will cut fuel until the engine gets back down to idle. Why waste fuel and backfire while you decel?

Or if you are accelerating up a hill, pedal to the floor, the TPS reporting WOT - wide open throttle ) but your rpm's are only 2500, it will inject extra fuel in to help you accelerate.

The TPS tells your ECM that your accelerator pedal is at an idle position. It will then try and regulate your idle speed. So if it's not getting one or the other, you're going to have idle issues, as you report.

Usually if the ECM isn't getting a tach signal it won't fire the injectors. It sounds MORE LIKELY to me you have a TPS issue.

You shouldn't be getting fumes in the cockpit. None. Have you checked for exhaust leaks? Furthermore, a fuel injected vehicle, does yours have a cat? With a cat it should be virtually impossible to smell ANYTHING.

If yours is more than 125K miles old it's time to replace the 02 sensor anyway. YOu'll get better mileage AND performance with a fresh one. That may cut down on your smells too.

HTH --- overall it sounds like your ECM is NOT getting all the correct information it needs.

[johnny haywire]

Hi Wade,

Thanks for the info. Here's what confuses me - my tach needle is frozen at about 700 RPM. From what I understand, the TPS has three settings, idle, 1400+ RPM and WOT. If it is relying on the tach signal to make these changes, then perhaps the signal for the tach is still being sent, just not registering on the tach itself? I was thinking the same thing, that the car wouldn't run if there was no tach signal.

The fumes in the cockpit are when the windows are open, I should have mentioned that. The car is running very rich and I had the cat removed while I was driving it home to Chicago from California due to the converter getting too hot (due to the rich mixture), so...even more rich fumes as a result of that.

The car only has 92K on it, but I think the O2 sensor wouldn't be a bad thing to replace.

i guess my thinking right now is that I should fix the tach before moving forward on other tests, but perhaps all the wiring is fine and the tach itself is just shot (so signal is being sent to ECU and sensors, just not showing on the tach).

I'm just wondering where to start. With the tach, then?

[Wade Nelson]

Personally I would start by getting my hands on the relevant section of the Factory Service Manual and diagnosing the throttle position sensor using a voltmeter. Or just throw a new one at it to see what happens. Then I'd take it into a muffler shop and have them seal up whatever leak the morons who cut your cat off left behind. After that I'd determine if it was still running in open, not closed loop, and diagnose why.

Are you planning to just rip other emissions parts off it without first diagnosing why they're not working properly? If so, I'll let someone else here pick up the ball of assisting you.

[johnny haywire]

Hi Wade,

I'm reading your mail - I own an FSM and have been going through the "EF" chapter and also reading up on the Atlantic Z tech pages regarding the FI system, but I'm still unclear about the tach signal relevance vs. actual tach needle operation. If the dead tach is affecting the loop, that needs to be done first, right? Because it could be sending a low or 0 RPM signal to the ECU?

I have no plans to rip anything off of the car; I hope to do the opposite, really. I don't want to start removing or replacing sensors (or using Atlantic Z "tweaks" to bypass a replaceable sensor) that don't need replacing. I want her to run right stock before any mods come into play. What I'd like to know the most is if the tach needle not moving is affecting the rest of the system.

As for the exhaust, I assisted with the cat removal, even bought the parts to patch in, and the guy did a solid job, I was 3 feet away from him the whole time, even set the lift arms to protect the frame before she went up. No leaks from the exhaust, just no cat, and I feel like I have a few sensors are stuck "on" that shouldn't be.

I just want to start in the right place and I want to be sure that if the tach needle not moving is screwing up the baseline for the system, then that needs addressing first.

Thanks for your thoughts - I owe you a beer!

[FastWoman]

Johnny, Wade's advice is spot-on for most modern vehicles, but our Z's are more primitive than he's giving them credit. With regard to the L-Jetronic system...

There's no connection between the tachometer operation and the ECU. Both are fed by the same signal from the (-) of the ignition coil (which connects to the "S" terminal of the tach and the #1 pin on the ECU connector). However, the tach doesn't even need to be present in the car for the engine to run. (I know this for certain because my dash was missing its tach for a while.) The ECU gets all it needs from that signal to the #1 pin. It has its own circuits for determining engine speed.

I'll also mention that the L-Jetronic ECU is completely analog and has no O2 sensor feedback, so your engine can only run in open loop mode -- the only operational mode this system has. As a result, your system might be running rich and not "know" it. To get your engine right, you have to make certain all the sensors are right, and you have to clear up any vacuum leaks. Even then, the ECU can drift in its calibration over the decades. These things tend to drift lean, especially on the '78 models, for some reason. (There are remedies for this that don't involve replacing the ECU.)

If you're running rich (strong fumes), do check the TPS, but especially check the coolant temp sensor and its connections. That connector tends to get corroded and crumbly, and if the connection opens up, your engine can run incredibly rich. Also check for a stuck (open) cold start valve or a malfunctioning thermotime switch.

Edited June 13, 2012 by FastWoman

[FastWoman]

Oh, finally, in '78 the cat was only required on California 280Z vehicles. If yours isn't specifically a California model, then you might be able to leave the thing off. Even if it is, it's quite possible nobody will know the difference. I've been asked about the absence of a cat on my '78 Z, and I simply explain that's how it was built. As I recall, the factory emissions sticker on the underside of my hood makes no mention of a cat.

[johnny haywire]

Hi FastWoman,

Many thanks for the clarification on the tach signal and the tips on where to start. I was curious as well about the O2 sensor (I hadn't noticed one). The car definitely doesn't know it's running rich ; ).

My plan of attack will be to replace all vacuum hoses first (and use the yogurt cup test for sure). I did a few of the hoses already, but not all. And then go through the sensors, starting with your suggestion - the coolant temp sensor. It may well be the cost start valve as well (or both). The fast idle on the car does not like to settle into low idle once it's warmed up. It will on a very hot day, but not until well after the coolant has reached operating temperature. Does that sound like a coolant temp sensor symptom? The temp gauge seems to be accurate, but that comes from a different sensor, right?

I'm also going to do the test on the TPS once I throw the new tach in (it's arriving tomorrow) so I can determine engine speed more accurately for the test/adjustment. Getting the old one out is going to be tricky as there is a full dash cover and it's overlapping the tach bezel a bit.

My particular Z is a California car - it's indicated on the under hood sticker (I should probably check the altitude sensor as well while I'm at it). I did change the plugs/wires/cap and rotor the other day and she runs stronger (mismatched plugs, worn rotor), but still very rich and the old plugs were sooty when I pulled them.

Thanks again for your tips and advice, very much appreciated. I'll be sure to post a follow-up on what I find! Should get a little wrench time in over the next few days.

Edited June 14, 2012 by johnny haywire

[FastWoman]

Fixing vacuum leaks is always a good idea, but you need to realize that will make your engine run richer, not leaner.

A long, lingering fast idle is probably a fault with the air regulator. It's got an electric heater inside that could be out. This is a troublesome part with which I seem to have lots of trouble myself. (I'm on my second used part now, and both have stuck open. I guess I'll have to break down and buy a new one.)

You're correct that the temp gauge and the EFI have different sensors, both of which are stuck in the front of the thermostat housing. The one-wire sensor is for your gauge, and the two-wire sensor is for the ECU. Note that the ECU's temp sensor and the thermotime switch are located right next to each other and have the same connector type. They will frequently get their plugs swapped. The thermotime switch (which has the larger body) should be connected to the plug that is wired in parallel with the cold start valve. If they're swapped, that could be the source of your high idle and rich running problem right there!

I'm afraid I know nothing about the additional California stuff. No need for altitude sensors where I live. We're at sea level!

Good luck tracking everything down!

[johnny haywire]

Thanks again, FastWoman, I'm going to check the connections on the temp sensors tomorrow and I'll let you know what I find. Good point about the vacuum leak corrections making it run richer. I hadn't thought about that.

The temp gauge works properly at present and I remember cleaning the thermotime switch contacts last year, so the connections are probably (but possibly not) correct. I kind of suspect my cold start valve may be leaking or my TPS is stuck in WOT mode. Well, all the sensors are suspect at this point. The previous owner's techniques to get it running were a bit dubious. I'll have a look at the air regulator as well when I do that.

But I did get a tach over the weekend! Excited to install that and get the car timed properly, then I'll test the TPS to see if it's working properly, and then on down the line with the rest of them. But first the battle of getting the dead tach out of a dash with a full cover (that overlaps slightly onto the gauge bezel).

Edited June 18, 2012 by johnny haywire

[johnny haywire]

Just a quick update - the new tach is installed and working! That was a bit of a chore, but it worked out in the end.

So, I got the car warmed up and then played with the TPS and I was able to change the idle speed by adjusting its position, so I'm thinking that the TPS is working fine. I have yet to pull the cold start valve to check if it's leaking (I suspect it is), but I did verify the connections on the thermotime switch and the water temp. sensor (I just received my color wiring diagram in the mail today). They seem to be connected correctly, and I even tried reversing them. With the connectors reversed, it would hardly start, and then would die immediately. Once back to proper position, the car fired right up and went into its normal (or what has been normal since I've owned it) high idle for 5 minutes.

Someone suggested that I pull the water temp sensor and clean it. Is it common for these to build up corrosion and not work properly? The pins on the sensor were crusty but I cleaned those and the pins on the thermotime switch. Still running rich, though.

Would the air regulator have an effect on richness? Sorry to be so needy, but is there a way to bench or quick test the air regulator?

I still don't have a good voltage tester, but that is up next on the list.

Thanks again for all your help!!

[FastWoman]

Hi Johnny,

I seriously doubt your water temp sensor could have that much crud built up on it. If it does, then you have bigger problems (clogged water passages and such). Even if it were covered with crud (which I doubt), it shouldn't affect temperature readings. Of course the crud on the electrical connections is another matter. Electrical connections do get crusty and do require periodic cleaning.

The air regulator wouldn't affect richness. It only affects idle speed. To semi-bench test it, pull the hoses off of it when your engine is cold, and look through it with a mirror. You'll see a little baffle that should be partially open. Now turn on the ignition without cranking the engine, and continue looking through the regulator. You should see the baffle slowly close as the bimetal strip is electrically heated. If this happens, then the air regulator is doing its job.

I hope that helps!