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Help with Throttle Position Sensor 78 280 Z


zeenubee

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Still trying to get this 78 running...old mechanic working on it for me ( I know very little)...have drained tank, PO seems to have cleaned it; new fuel pump (walpro); gaslines blown out; fuel pressure tested at filter(new) at about 45#; he says he checked and cleaned(?) all electrical connections; new fuel injectors (ebay) and seals; AFM cleaned. Now he says he checked the TPS, cleaned contacts and it runs "better" but not like it should...aftermarket TPS seems to be only available now thru the zcar place in Ariz at $125.+ Any suggestions before I throw more parts at this thing?

Edited by zeenubee
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Did he say that you needed a new TPS or are you assuming? You might be misunderstanding what his remarks mean. There are only three positions for the throttle valve (aka position) switch - idle, wide open throttle and in-between. If he ran the check in the FSM and got the right values, then you don't need a TPS. Don't buy a new switch. It's not a variable sensor like on some modern engines.

You might need a new mechanic. It sounds like he's guessing and not using the FSM or EFI Handbook, because 45 psi fuel pressure is normal for some modern engines but is way too high for these engines. 36.3 psi is the proper number and he should have known that. High fuel pressure would cause the engine to run rich and the exhaust to smell like gasoline.

Old skilled mechanics are sometimes over-confident and will try to figure things out by looking and testing, instead of doing things the right way.

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Is your mechanic using the FSM and the EFI supplement?

He would have also done better testing the fuel pressure after the FPR to see if it was working at 36psi. He "cleaned the AFM" but did he do the tests in the FSM? It sounds like he is using his knowledge of other EFI systems to work on this one, but the Datsun system is a early system and you need to take that into account.

Here is a link to the manuals XenonS30

Chas

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Giving the mechanic the benefit of the doubt, that 45 psi could have been static pressure. But I agree the L-Jetronic is an unusual EFI, and it is extremely important to refer to the FSM, no matter how much you may think you know about cars in general.

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I did provide him with a copy of the FSM and the EFI Bible...but I cannot tell you he referenced them...I do know that he didn't do the values check at the ECU. I do believe you might be right when you say he may be overconfident in his own knowledge.

He cleaned the contacts on the TPS and that is when he got improvement...I have said all along that I thought it was an electrical problem. And, yes he asked me to get a new TPS. I guess I will have it towed home and go through a the electrical connections my self with the EFI manual and an multimeter and emory cloth and deoxit. I will have him re-check the fuel pressure after the FPR and go from there...thanks for the comments! This is a great site...wish I could get this old timer to look at it!

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You can visually check the TPS yourself by removing the cover and operating the throttle rod. at idle, one set of contacts are closed, then as you open the throttle they open (no contacts touching) then at about 2/3 open the other set of contacts close. You don't need a mechanic to test that. And if the contacts or connection needs cleaning, you can spritz them with DeOxit (you DO have some, right?) ;)

I'm leery myself of "former Datsun mechanics." There is one near me, and I asked him to look into the high idle I get when warm. (vacuum leak?) The first thing the guy told me is "your Idle air motor isn't connected and the MAF was changed. The computer was changed too."

HUH? It's a 40-year-old engine with (early) EFI..

It reminds me of when I asked the guy at the e-check station to run the sniffer in the exhaust. The first thing he said was, "where is your OBD port?" An older guy had to tell him there wasn't one...

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+1 on what others have said.

With the FSM/EFI Bible & a simple voltmeter you will be able to check all the engine harness/external sensors. Even if you aren't super mechanical you can do this. You may have to ask some questions on here (don't be afraid to ask simple questions. We are great with those. :)) to get it figured out, but this forum has some very helpful peeps.

Absolute worse case, you learn more about your car and have a better idea of where to steer your mechanic should you decide to have him or someone else look at it.

Len

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Thanks guys...brought the car home...had a very hard time...would run ok for a minute or two but no real power...6 miles and I had to tow it the last too.....I'm going to throw this idea out and see what you think...the car had been sitting for about 8 years and had a lot of mice in it...could the exhaust be partially stopped up and that rob the power?

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It's more likely the mice chewed some of your wiring. I'd look for that first.

BTW, be very careful with any mouse poop you find. Mice can carry the hantavirus, which is deadly. Wear a mask, wash your hands, and shower after handling mouse debris.

Edited by FastWoman
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What does it do when it lacks power? Does it cough and buck and backfire, or just "bog" when you give it gas, or blubber and choke like it's flooding? With 45 psi you might have been running rich with fouled plugs, and finally fouled too many on the way for the engine to run. Coughing and back-firing is typical of lean, fouling plugs can happen if it's rich. "Bogging" could be timing.

I would pull the plugs to see what the few miles of driving did to them (keep them in order 1 - 6 in case you have cylinder-specific problems), test all of the EFI components from the ECU connector that you can figure out from the FSM so you'll know from the start what the ECU sees, and check the timing (pretty common to have someone turn the distributor to see what happens and get the timing either way retarded or way advanced).

Didn't realize it was that bad. Good luck.

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He's correct. Too may times back in the days of carburetors and points have I seen people tuning the engine for "power" by ear by turning the distributor, then finding out the gas mileage went way down. Later, with the timing light, they find out how far off things were.

You don't have to go out and (blow your whole Christmas budget) on engine diagnosis tools and hand tools, but if you can, things like a timing light, an analog voltmeter (or cheap engine analyzer,) digital voltmeter, test lamp/buzzer, should go on your list of things to get when you can afford it. Maybe a neighbor has the stuff, or maybe you can borrow or rent it from the local Auto parts store. Check your local second-hand store! You find the neatest old stuff there (take a notepad to record ideas for stuff to buy later.) You will figure out how much or how useful a tool is for you and set its priority on your "car tools to acquire" list.

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in reply to Zed Head. et al...it doesn't cough or backfire...it bogs down, no smell of gas that I can tell...I do think he messed with the distributor because he replaced the cap...small crack. I will re-check the fuel pressure. I did notice in the efi bible that the picture of the FPR shows two inlets....one side of mine is blocked off...don't know who did.. this may be PO, but the car ran fine when I bought it in 2002. I have most hand tools, but don't have a timing light, think I have a cheap multimeter. I will start with the efi bible and check the values if I can figure out how to do it...seems pretty simple...then I will check the plugs as suggested...thought maybe the mice had partially plugged the exhaust with soybeans as I find them everywhere in my garage...they used the air filter for their main bathroom....no evidence of chewed wire yet, but I haven't been up under the dash yet. Thanks guys! will let you know more over the weekend hopefully.

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If you have anything blocked off on your FPR, then you have a problem. I'm guessing your return line is blocked off, and the fuel rail is running at the full output pressure of the fuel pump, which would be in the neighborhood of 45 psi. Instead, it should be running in the neighborhood of 30 psi. Higher pressure means too much fuel injected. That's likely why your engine is choking -- or at least one of the reasons.

Take a look at where your steel fuel lines enter the engine compartment from under the car on the passenger side. One of them will go to the fuel filter and is your supply line. There should be another one right next to it. I bet it's blocked off. There should be a fuel line between the fuel pressure regulator and that steel line to carry return flow back to the tank. Before hooking it up, blow some air through it to make certain it's not blocked with rust and debris. If it's blocked, you'll still have the same fuel pressure problem.

I agree with Euro: A picture would tell us a lot.

Edited by FastWoman
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Might be a good idea to put the model and build date in your signature.

The 78 has the FPR with one inlet (N47). If one is blocked??? That must be the return line or the vacuum line. Either way its getting to much fuel pressure and that will make it run rich.

The fuel injector is in simple words an on/off valve. The size (in the process industry we call it the Cv value) of the injector is based on the pressure drop across the tip. Google "Cv pressure drop" or "Cv values" in metric its called Kvs and wikki will give you a good read on the relationship between flow and pressure.

The FPR maintains the pressure drop across the fuel injector at 36psi. The ECU does all its calculations based on this and if its out of spec, it will effect everything the ECU does.

BTW: The two inlet version (N42) was used upto juli 77. In August 77 they they changed over to the fuel rail in your model.

Chas

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Might be a good idea to put the model and build date in your signature. I just noticed your first post again "getting the 78 running".

The 78 has the FPR with one inlet (N47). If one is blocked??? That must be the return line or the vacuum line. Either way its getting to much fuel pressure and that will make it run rich.

The fuel injector is in simple words an on/off valve. The size (in the process industry we call it the Cv value) of the injector is based on the pressure drop across the tip. Google "Cv pressure drop" or "Cv values" in metric its called Kvs and wikki will give you a good read on the relationship between flow and pressure.

The FPR maintains the pressure drop across the fuel injector at 36psi. The ECU does all its calculations based on this and if its out of spec, it will effect everything the ECU does.

BTW: The two inlet version (N42) was used upto juli 77. In August 77 they they changed over to the fuel rail in your model.

Chas

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OK...will check the pic in efi bible, take a pic of my set up, and check the return line also...wont be able to do this til sun or mon....thanks!!!!.....one ,more thing to learn...how to post a pic here (LOL) :)

Click Go Advanced. Scroll down to Additional Options. Click Manage Attachments, a small window will open. Click Add Files, then click Select Files. Find your picture from your camera or your hard drive, then in that box click Open then it'll close and go back to the 1st small box. Click Upload Files. They should appear on the bottom left of the light blue bar at the bottom. Click Done. That's how mine works.

post-24724-14150826940471_thumb.jpg

Edited by siteunseen
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Pictures look great. Well-sized.

Looks like someone installed a 75-77 FPR on your 78 fuel rail. It should work fine, but that's why it's not attached properly and has the blocked off port. Not the safest set-up either since the only thing holding the FPR on is the hose clamps on the inlet and outlet ports (unless there's a bolt that I can't see in the picture). If the clamps loosen it could pop right off and the engine would get dowsed with gasoline. Check those clamps and see if you can get a bolt in to the bracket.

The part of your situation that seems most important is that the engine died and you had to have the car towed. If it was a simple tuning problem, you should be able to drive around with low power all day. Does the engine start now? If so, it could be a heat-related problem, if not, maybe something broke unrelated to the low power problem.

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FWIW I dug up a couple diagrams that ought to help. the first is the fuel pressure regulator, and shows you where the different connection go, and the second is entire fuel injection rail, to show where the hoses and metal tubes go.

These are both for the 1978 L28E engine.

post-2169-14150826952658_thumb.jpg

post-2169-1415082695293_thumb.jpg

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Yes, the FPR looks fine. Mine is a '78, so I didn't know about the extra inlet in the earlier models. (I'm afraid my '75 is but a distant memory from an earlier life.)

I do see your EGR equipment is missing, and you have a BPT valve control tube hanging open. It's the small metal tube ending almost dead-center in your 4th photo. I suspect exhaust leaks through the tube. This wouldn't keep your engine from running properly, but it could be a source of carbon monoxide in the cabin.

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