1976 280z engine runs rough then dies, will not idle.

[Reptoid Overlords]

I had a vacuum leak at the intake/exhaust manifold gasket several years back. It caused what seemed like lean misfire symptoms.

I sprayed throttle body cleaner along the mating surface of the intake manifold and head with the car running and found that the idle would surge where the throttle body cleaner entered into the intake leak.

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[Dave WM]

I don't think he has a vacuum leak in the conventional sense. It something wrong with the FPR failing to regulate. The most likely suspect is a problem with the vacuum source. I find it odd that there is NO change in pressure with the engine running. Compound that with what appears to be a over RICH condition point away from normal un metered air issues.

[Dave WM]

FYI I just ran another test on the same kind of rail that you have (the two piece rail) and same results as my video, 30 psi at 17" 38 with no vacuum. I was going to try to use the ZX rail in my test run but it was just such a pita getting it installed, I found the old rail a bit easier. Plus I used some really nice injector spacers off a maxima they work much better than the fiddly earlier stuff I have.

[mbz]

12 minutes ago, Dave WM said:

Maybe you should take a pic of some of the vacuum hookups, specifically the FPR and the EGR.

FPR to Manifold

EGR Valve Solenoid to EGR Valve:

EGR Valve Solenoid to metal line going to throttle body:

Other end of metal line from EGR Valve solenoid:

Vacuum line connected to Throttle Body:

From the throttle body, the vacuum line connects to a " T " which splits it to the distributor and also to the charcoal canister.

 

 

 

[mbz]

OK re-reading service records from PO's mechanic as to the last work done on the car.

Service notes state:

"reseal fuel injectors, replace mount holders and all fuel rail hoses - leaking vacuum and fuel cold" (it ends abruptly but I'm assuming this means fuel cold start valve - but not sure)

It further states:

"reseal thermostat upper housing"

 

I think a phone call to the PO's Mech is in order to get some clarity on the "leaking vacuum" that was repaired.

 

[Dave WM]

that all looks correct. Disconnect the line from vacuum line from the EGR for now, but remember it could still be stuck open, or a piece of carbon jamming it open resulting in an air leak. The FSM test is to idle the engine and then stick your finger under the EGR diaphragm and lift it up, which should open the valve and cause the engine to stumble. But since yours is not running right its not a good test. The only way to know for sure is to unbolt the EGR (try not to break the bolts, lots of PB blaster and patience) and examine it. Its a pretty simple device you will know if its stuck open.

 

[mbz]

1 minute ago, Dave WM said:

The only way to know for sure is to unbolt the EGR (try not to break the bolts, lots of PB blaster and patience) and examine it. Its a pretty simple device you will know if its stuck open.

Yes it looks pretty simple, and probably best to inspect anyway.

However I am very keen to find out what the "leaking vacuum" was that the PO's Mech referred to in the last repair notes from July 2018.

I'll put a call into him tomorrow AM and see if I can get any relevant info.

[Dave WM]

Clearly you need to test the vacuum the finger test is easy and quick, the gauge will be better.

 

[mbz]

2 minutes ago, Dave WM said:

Clearly you need to test the vacuum the finger test is easy and quick, the gauge will be better.

I'll pick up a vacuum gauge and a brake bleeder (can pull a vacuum) as well tomorrow. I'll need them both anyway and HF is having a big sale. Should be able to get them both for about $30 bux including tax.

Edited April 13, 2019 by mbz

[Dave WM]

The problem is the leak has to be massive to account for the FPR not working. I mean HUGE, and you would hear it anywhere but the EGR. I use a rubber tube held to my hear and poke around suspected places for vacuum leaks. The smoke test is the best way to find difficult cases. This assume the NEW FPR is working correctly, that is if you apply a vacuum it should reduce pressure. a brake bleeder like in my video works well but in a pinch you can just get a length of hose attach it to the vacuum port on the FSR and suck on it. you should be able to get at least a decent enough vacuum to see the effect.

 

[mbz]

1 minute ago, Dave WM said:

that is if you apply a vacuum it should reduce pressure. a brake bleeder like in my video works well

I'll pick one up (brake bleeder) tomorrow and test the FPR.

2 minutes ago, Dave WM said:

The problem is the leak has to be massive to account for the FPR not working. I mean HUGE

Again, that's why I want to talk to the POs Mech and find out what his notes meant in regards to the "leaking vacuum".

[Dave WM]

I can assure you the water temp switch is bad, everyone I have tested (several) were bad. However other than a slightly rough idle when cold it would not account for this magnitude of problem. I made one out of some old parts and a new heat activated switch some time ago for my car. Beware of buying one, you need to make sure its correct, N.C. or N.O. I don't recall but the ones they sell are for fans which I think tend to be NO until operating temps are reached. IIRC you need a NC until operating temps are reached.