Engine leans out over 2400 rpm

[Roberts280Z]

The FSM seems adamant that 36.3 psi is required, in both the discharge pressure check and full-load requirement check.

[Zed Head]

25 minutes ago, Cruzzar said:

I have pulled the cover and can visually see that the idle points are open when the engine goes lean.

The best way to test the EFI system is with a meter at the ECU connector.  What the computer sees is what matters.  Your fuel pressure readings looked in the ballpark, but 32 at idle suggests low intake vacuum.  The EGR system is often the culprit there.

 

[Zed Head]

3 minutes ago, Roberts280Z said:

The FSM seems adamant that 36.3 psi is required, in both the discharge pressure check and full-load requirement check.

I think that when he said 32 he meant with the engine running and intake vacuum applied to the FPR.

[Roberts280Z]

I have a Ben Watson book on Bosch FI systems, which states in general that the pressure at idle should be 30-35 psi, and should increase by 5-10 psi with higher manifold pressure (i.e. lower vacuum).

I think I recall a situation where my fuel flow was restricted due to the fuel pickup in the tank being partially clogged. I temporarily helped the situation by blowing back through the pump intake line into the tank.

[Zed Head]

It's kind of funny how automotive mixes up so many different types of measurement, but you can calculate the fuel pressure drop from the intake manifold vacuum, and vice versa.

A good running engine will have about 16 inches of vacuum at idle.  Convert that to pressure and it tells you how much drop the FPR should cause.  About 7.8 psi.  36.3 - 7.8 = 28.5 psi.  14 inches gives 29.4 psi.

32 psi correlates to 8.8 inches of vacuum.  Not very good.  Could be valves, timing, vacuum leaks.

Just more clues to work with.

https://help.summitracing.com/app/answers/detail/a_id/5306/~/engine-vacuum

https://www.onlineconversion.com/pressure.htm

Edited May 22, 2021 by Zed Head
wrong number

[Cruzzar]

Zed Head, In regards to the EGR, I have pulled a vacuum on the EGR valve and it holds a vacuum, I have plugged the vacuum port for that hose with still no change in engine performance, I even plugged both vacuum hoses going to the fuel canister with no change. Is it possible that the EGR valve is stuck in the open position and causing a vacuum leak? At idle when warm it pulls 18 inches. of Hg. so I think the pressure regulator is receiving a strong signal. I have not checked the PCV (I don't even know where it is) is it possible that it is stuck? I can pull the hose coming from the top of the valve cover and it will develop a pretty strong vacuum. If there was a large vacuum leak wouldn't it cause a hard start and a lean condition all the way through the rpm range? Engine had good compression (152 low-160 high)

[Zed Head]

2 minutes ago, Cruzzar said:

At idle when warm it pulls 18 inches. of Hg.

So, you have some inconsistencies, which are probably clues.  I just reread your first post and see the vacuum reading.  That's a good number.  Your fuel pressure at idle should be down around 28 psi.  I've had that on mine.  Maybe you have a bad fuel pressure gauge.  Not uncommon.  Maybe you have bad FPR.  If it's original the diaphragm can stiffen up.

There's a big hose under the intake manifold that connects to the side of the block.  That's the main PCV hose.  They can fail.

Can you describe the problem better.  "It accelerates up to about 2500 rpm and then it falls flat on its face. "  is not very much.  Is it always right at 2500?  Does it make any noises when it fails?  Does it recover immediately at 2300 RPM?  Stuff like that.

The suggestion to watch the pressure gauge while driving is a good one.  Might be something weird like losing pump power.  Under acceleration fuel pressure should stay up around 36 psi.

 

[zclocks]

Location of PVC. If in question easy to remove and clean with solvent. If you can shake it and it rattles then it's probably ok.

[Zed Head]

Another random thought.  If the AFM is old the carbon trace could be bad.  Maybe there's a certain point of the vane travel where the AFM sends bad data to the ECU.  One of the AFM tests is to run it through its travel and watch resistance or voltage.  Can't remember which.

Found it.  It's in the FSM but not the Guidebook.  Page EF-51 in the 76 FSM.

Edit - you can clean the trace with contact cleaner.  Hose it down, move the vane.  See what happens.

Edited May 22, 2021 by Zed Head

[siteunseen]

30 minutes ago, Zed Head said:

Another random thought.  If the AFM is old the carbon trace could be bad.  Maybe there's a certain point of the vane travel where the AFM sends bad data to the ECU.  One of the AFM tests is to run it through its travel and watch resistance or voltage.  Can't remember which.

Found it.  It's in the FSM but not the Guidebook.  Page EF-51 in the 76 FSM.

Edit - you can clean the trace with contact cleaner.  Hose it down, move the vane.  See what happens.

I bought a '76 that wouldn't go beyond idle. Spider web on the afm vane had it stuck. One of my all time lucky breaks. Probably none since then and that was '86, '87.

 

[Zed Head]

17 hours ago, Cruzzar said:

parked it under a large carport in 1995

Spiders.  Chipmunks.  Mice.  Carports are outside.

Could even be throttle linkage.  Make sure all the moving parts move like they should and that there's nothing blocking air flow.  Check the inlet tube to the air filter too.

[Captain Obvious]

Interesting... Pretty much everything sounds like it's working OK, but the engine is starving for fuel once you get above idle.

So to answer some questions: Yes, it is possible for the EGR to be stuck open even if there's no vacuum applied. But that would cause an issue across all operation conditions, especially idle. If the EGR is stuck wide open, I doubt you'd be able to get a steady idle at all. Same for the PCV if it were stuck open.

Sounds like a classic fuel capacity issue, but your fuel pressure up at the rail is OK.

Maybe your fuel injectors are partially clogged? They can pass enough fuel at idle (because the engine just doesn't need that much fuel at idle), but when you're taxing them to pass more fuel than that, they can't. There are little screen filter cups on the inlets to the fuel injectors. Maybe they are partially plugged with crud? Maybe the injectors themselves are plugged, or sticky, or both?

10 hours ago, Cruzzar said:

at times I can get the rpms up to 3500 if I advance very slowly indicating to me that the injectors have the flow capacity and the a/f ratio is rich enough to maintain combustion.

I'm not sure your conclusions are valid. You're running a tiny bit rich at idle (12.7 - 13.0) which is fine, but just because you can sometimes nurse it up to 3500 doesn't mean you've really got the flow capacity. An engine with no load will "run" with an extremely lean mixture. What's the A/F ratio when you manage to nurse it to 3500?

To me, it sounds like you're a little rich at idle, and as the RPM's go up, you get a little leaner, and a little leaner, and a little leaner, until it's finally so lean that it just won't run at all.