AEM Gauge shows lean at WOT. Wondering why?

[1987 Porsche]

Juanbeane: Bursch cat test pipe. Sensor is in bung in picture.

[1979-930]

When checking fuel pressure have you dead headed the pump to see how quickly it goes to 7 Bar? I think it's 7 Bar anyway, It's been a couple years.
I had a similar issue with a bad pump. The car ran and drove fine. After installing headers, bigger turbo and other mods the car would lean out bad on boost. A AFR gauge went in with the mods. When setting the adjustable WUR I did dead head the pump, as suggested, and it lazily got to like 6 or 7 bar and thought that everything was ok.
Found out with some tests the front pump was not pumping enough volume.
Installed the new front pump and checked pressures again. This time when I closed the valve and dead headed the pump the gauge snapped to full pressure. Problem solved.

So dead head the pump while the gauge is attached and see how fast it hits max pressure. It should be instant. If it slowly raises then you have a weak fuel pump that can't keep up.

[1987 Porsche]

Derrick

Thanks for the reply. Can you explain the steps to dead head the pump? Is it the same as the system pressure test, but keeping an eye on how fast it climbs?

Lou

[boyt911sc]

Originally Posted by 1979-930

When checking fuel pressure have you dead headed the pump to see how quickly it goes to 7 Bar? I think it's 7 Bar anyway, It's been a couple years.
I had a similar issue with a bad pump. The car ran and drove fine. After installing headers, bigger turbo and other mods the car would lean out bad on boost. A AFR gauge went in with the mods. When setting the adjustable WUR I did dead head the pump, as suggested, and it lazily got to like 6 or 7 bar and thought that everything was ok.
Found out with some tests the front pump was not pumping enough volume.
Installed the new front pump and checked pressures again. This time when I closed the valve and dead headed the pump the gauge snapped to full pressure. Problem solved.

So dead head the pump while the gauge is attached and see how fast it hits max pressure. It should be instant. If it slowly raises then you have a weak fuel pump that can't keep up.

Derrick,

You can not compare a 930 CIS with two- FP with a normally aspirated CIS with a single fuel pump. These are two distinctly different CIS engines. The tests and parameters are night and day difference. Your 930 has a system fuel pressure between 90~100 psi. while Lou’s is only 65~75 psi. FYI.

Tony

[1979-930]

Originally Posted by 1987 Porsche

Derrick

Thanks for the reply. Can you explain the steps to dead head the pump? Is it the same as the system pressure test, but keeping an eye on how fast it climbs?

Lou

Yes, When you close the return valve during the pressure test the gauge should snap to full pressure. If it climbs slowly the pump is weak.
Fuel filter could be clogged too.

Originally Posted by boyt911sc

Derrick,

You can not compare a 930 CIS with two- FP with a normally aspirated CIS with a single fuel pump. These are two distinctly different CIS engines. The tests and parameters are night and day difference. Your 930 has a system fuel pressure between 90~100 psi. while Lou’s is only 65~75 psi. FYI.

Tony

The pressure should still climb rapidly to max pressure. A weak pump or clogged filter in any fuel system will cause a lean condition at full throttle.

[SkiVT]

May be a dumb question but what did the cat test pipe replace? I only ask because the PO for my car did some creative emissions changes that tied a cat, which is not standard on a euro car or your year, to the wur return fuel line. When that plumbing was disconnected, I had no enrichment. Have you owned the car throughout? Seems very strange the leaning shows up after all the engine work you have completed.

[1987 Porsche]

So the cat bypass pipe replaced the original catalytic converter. I have owned the car for about 5 years but never did I have an AFR gauge tied to it. It could of been doing this prior to the rebuild but I wouldn't have known. Today I pulled the CIS boot to do a thorough smoke test with a smoke machine I built. Hopefully something will show up. I will keep you guys posted.

[SkiVT]

My mistake. I thought the cat started in 1980 but that was when they added Lambda and a better cat. Hopefully the smoke machine will find something.

[SkiVT]

Is it possible the decel valve is bad? I thought I read it closes off vacuum at specific points. If it is allowing some vacuum through in the wot range, the wur will not fully richen with pressure staying up a bit. You tested vacuum to the wur for cold start. Maybe testing the decel ports is in order.

[wreckah]

How does the car feel at WOT?
if the AEM gauge is correct, then WOT should feel really slow when AFR's are above 14...the car should feel like 100hp instead of 200hp. I have driven my 3.0L SC with bad CIS system, and AFR's not rich enough (14, 15), and it feels completely flat. It feels broken. A healthy SC should have a lively acceleration, building power from 2000, becoming very nice at 3500rpm, and still crescendo to about 5000 rpm and then plateau/taper off.

just trying to exclude the AFR readings from the investigation.

If AFR numbers are correct, and the car does indeed feel slow, you have a problem with fuel delivery i think. Injectors were cleaned, so we maybe can exclude those...WUR is fine too. Then we have fuel pump and fuel distributor as main suspects? Has the car been sitting for a while before you got it?

[1987 Porsche]

Conducted the smoke test today using Tony's method and noticed a hairline crack on the seam of the airbox near the number one intake pipe. A small amount of smoke was seen coming out of this area and it definitely would not have been found by just visible inspection. Everything else was good. Since it is reachable I decided to seal it up with some JB weld. I'll give an update tomorrow. Hopefully this is it and the JB weld works.

[1987 Porsche]

Wreckah: car does feel strong at WOT. Not sluggish at all but I do believe it could be better. We will see how it is tomorrow

[boyt911sc]

You have to verify and confirm the absence of significant source/s of unmetered air. Not being able to find one does not mean you don’t have any. A pressure test with a smoke generator will tell you if the system is free of air leak source/s or not.

People have been using AFR (air to fuel ratio) meter blindly. The tool is very efficient and reliable if used correctly.

Tony

[AndrewCologne]

Originally Posted by 1987 Porsche

So I just installed my AEM gauge in my 78 911. I adjusted the warm idle mixture to 13.5 and have my idle set at 950. During wide open throttle or hard acceleration I notice the gauge climbing to the 15's indicating the car is leaning out rather then getting richer. Why would this happen?

After adjusting the mixture, .. didn't you forget to pull the allen wrench again out of the FP?
This sometimes even happens to mechanics with 30 years of 911 experience :-)
Cause then the Air sensor plate can't move freely upwards and the plug in the FD won't reach the position where enough fuel will be provided at WOT.

[1987 Porsche]

I always took the Allen key out after each adjustment.

[1987 Porsche]

So, I sealed up the hairline cracks alongside the airbox. Re checked with smoke and all seemed good. Lean condition at WOT is still present as well as rough cold start. Did replace the AEM Gauge with another one provided by the company, still same lean numbers at WOT. Whats your thoughts on what to do next? Here are mine:

1. Conduct a fuel test putting the injectors in the bottles to check for even fuel flow and pattern. (They were cleaned by Mr. Injector after the rebuild)
2. Replace fuel filter. But this should be good because system fuel pressure was in spec, right?

At this point, I think its a fuel issue since there are no present vacuum leaks, cold and warm pressures are in spec, WUR was rebuilt, new spark plugs, cap, rotor, and wires.

What am I missing? Thanks

Lou

[pampadori]

Any chance you can video the gauge as you run it a bit at wot?

[1987 Porsche]

Today I got a chance to test the injectors. I placed them in baby bottles, bypassed the relay, and lifted up the air flow sensor plate. Each injector had a good spray pattern, no leaks, and consistent volume across all six. I'm going to check the decel valve tomorrow. Im also going to disconnect the vacuum line to the WUR, tie that line into my vacuum gauge which I will monitor from inside the car (long vacuum hose) and go do some WOT pulls to see if the vacuum drops off.

[Nux]

Originally Posted by 1987 Porsche

So, I sealed up the hairline cracks alongside the airbox. Re checked with smoke and all seemed good. Lean condition at WOT is still present as well as rough cold start. Did replace the AEM Gauge with another one provided by the company, still same lean numbers at WOT. Whats your thoughts on what to do next? Here are mine:

1. Conduct a fuel test putting the injectors in the bottles to check for even fuel flow and pattern. (They were cleaned by Mr. Injector after the rebuild)
2. Replace fuel filter. But this should be good because system fuel pressure was in spec, right?

At this point, I think its a fuel issue since there are no present vacuum leaks, cold and warm pressures are in spec, WUR was rebuilt, new spark plugs, cap, rotor, and wires.

What am I missing? Thanks

Lou

I believe you're missing EFI

[1987 Porsche]

LOL. CIS has me losing my mind.