Lean idle on warm start

[NCGermerican]

Happy Thanksgiving folks! I'm trying to figure out a warm start lean condition on my 87 951. It starts and idles great when cold. It does start right up when warm, but it idles a little rough for about 45 seconds. My wideband shows over 17 and then gradually drops down to 14.7.

Theory #1 - Vapor Lock. I do smell a slight fuel vapor in the vacuum lines, but there's no fuel dripping when i pull the vacuum lines and they are dry. I used a mighty VAC on the FPR and Dampener vacuum ports and they both held 15 hg with no drop. I swapped out the fuel dampener just to see if anything changes but....nope. I also did a fuel pressure test:

FPR: Delphi 3.0 bar (about 2 years old and maybe 1,500 miles)
Fuel Pump: Bosch (about 3 years old, replaced tank sock and fuel filter (Mahle) at same time)
Plugs: confirmed 4.5 ohms on each injector and were cleaned/tested about 2 years ago

Readings
Jumpered (F9 DME relay): 36PSI / 2.5 bar
Engine Running with vacuum: 34PSI / 2.3 bar
Engine Running without vacuum: 40PSI / 2.8 bar
1 Minute leak down: Actually increased to 36PSI / 2.5 bar
20 Minute leak down: increased to 38PSI 2.6 bar and held pressure
**It's now been over 30 minutes and still at 38 PSI

1) That seems low to me for a 3.0 bar FPR. 2) I don't seem to have any drop in pressure, but the engine wasn't super hot either (I don't like messing with gas on a hot engine). Wouldn't the pressure need to drop for it to create space for vapor?

Theory #2 - could it be a bad DME temp sensor (Bosch and about 2 years old)? To avoid breaking out the multimeter, could I use my F9 DME w/ ODB diagnostics to see if it's bad? For instance, it shows "coolant temp", but does that mean from the DME temp sensor or the sending unit to the gauge?

Theory #3 (just spitballing), could the thermo valve that controls the evap have anything to do with it? I'm just trying to think of anything that is influenced by engine or coolant temperature.

My o2 sensor is Bosch and brand new when I replaced the motor back in August. When I check it on the F9 ODB program, it pretty much matches my wideband.

This one has been baffling me and is seeming to throw ChatGPT for a loop as well.

[Tom]

A 3 bar FPR is not stock for the motor, so what chips are you running? Have you tried a stock FPR with stock chips? 2.8 bar with the vacuum disconnected does seem a bit low -- is it low like that when cold? The O2 Sensor takes a while to warm up and start regulating fuel. It's possible you have a vacuum leak causing the lean condition, but it's just small enough to be masked by the O2 closed-looped system once it kicks in (and masked by the cold-start enrichment when cold). Does your car tend to run on the rich side under heavy boost? intake air leaks cause the motor to be lean under vacuum as it suck in unmetered air, but causes the motor to run rich under boost as it blows out metered air...

Is this suddenly new behavior or has it always been this way? The DME has no way to see the gauge temp, so I have to assume the F9 DME is reporting the blue (DME) temp sensor.

[johnb]

I agree with Tom that a lean condition being masked under certain conditions is the most likely. I'm not sure if this is useful to know but the stock chip targets about 14.4 at idle for normal operating temps. The o2 sensor leans it back to 14.7 but will definitely mask lean conditions when hot. When you turn the engine off the o2 sensor cools down faster than the coolant in the block (which is what the DME relies on).

[NCGermerican]

A 3 bar FPR is not stock for the motor, so what chips are you running? Have you tried a stock FPR with stock chips? 2.8 bar with the vacuum disconnected does seem a bit low -- is it low like that when cold? The O2 Sensor takes a while to warm up and start regulating fuel. It's possible you have a vacuum leak causing the lean condition, but it's just small enough to be masked by the O2 closed-looped system once it kicks in (and masked by the cold-start enrichment when cold). Does your car tend to run on the rich side under heavy boost? intake air leaks cause the motor to be lean under vacuum as it suck in unmetered air, but causes the motor to run rich under boost as it blows out metered air...

Is this suddenly new behavior or has it always been this way? The DME has no way to see the gauge temp, so I have to assume the F9 DME is reporting the blue (DME) temp sensor.

Sorry - I should have added that in. I have the Lindsey Racing "Max Performance" chip from when I had the "LR 300hp kit". At that time I was on the stock turbo/WG/cycling valve. I now have a Raptor X / Tial / MBC and still using the same LR chip in my F9 DME.

I suppose it could be a vacuum leak, but outside of the warm start lean idle (that clears up after about 45 seconds), the car runs fine. Cold start and idle are just about perfect with AFR's right at 14.7 +/- .3.

[NCGermerican]

I agree with Tom that a lean condition being masked under certain conditions is the most likely. I'm not sure if this is useful to know but the stock chip targets about 14.4 at idle for normal operating temps. The o2 sensor leans it back to 14.7 but will definitely mask lean conditions when hot. When you turn the engine off the o2 sensor cools down faster than the coolant in the block (which is what the DME relies on).

I'll run a smoke test on it. I just hope it's not a super small leak that only shows itself when the car is warm.

[johnb]

I agree with Tom that a lean condition being masked under certain conditions is the most likely. I'm not sure if this is useful to know but the stock chip targets about 14.4 at idle for normal operating temps. The o2 sensor leans it back to 14.7 but will definitely mask lean conditions when hot. When you turn the engine off the o2 sensor cools down faster than the coolant in the block (which is what the DME relies on).

I'll run a smoke test on it. I just hope it's not a super small leak that only shows itself when the car is warm.

What does it do with the o2 sensor unplugged, cold and hot? It should be a lot quicker to test that that doing a smoke test.

I don't know what the target AFRs would be for your chip but if running without the o2 makes it idle leaner when hot then I would take that as a strong indication of a vacuum leak.

[NCGermerican]

I agree with Tom that a lean condition being masked under certain conditions is the most likely. I'm not sure if this is useful to know but the stock chip targets about 14.4 at idle for normal operating temps. The o2 sensor leans it back to 14.7 but will definitely mask lean conditions when hot. When you turn the engine off the o2 sensor cools down faster than the coolant in the block (which is what the DME relies on).

I'll run a smoke test on it. I just hope it's not a super small leak that only shows itself when the car is warm.

What does it do with the o2 sensor unplugged, cold and hot? It should be a lot quicker to test that that doing a smoke test.

I don't know what the target AFRs would be for your chip but if running without the o2 makes it idle leaner when hot then I would take that as a strong indication of a vacuum leak.

I didn't even think of that. I'll have to check that out tomorrow when I can take it out for a spin.

[NCGermerican]

So I've had a ton of stuff going on since I started this thread and haven't had time to test cold/warm AFR's without the O2 plugged in (even though it would take me 5 minutes).

I wanted to bring up something I noticed as well. When I replaced the motor in August, I replaced the AOS seals to the block. One thing I did NOT replace is the o-ring on the AOS cap. I looked through the intake runners last week and noticed an oily film on top of the AOS. All hose clamps are tight, so I doubt the leak is coming from there. However, is it possible that the seal on the AOS cap is leaking and if so, could that be the cause of the residue and possibly be enough of an unmetered air leak to throw off my AFRs?

I'm grasping at straws here but don't want to take off the intake again if I don't have to.

[johnb]

So I've had a ton of stuff going on since I started this thread and haven't had time to test cold/warm AFR's without the O2 plugged in (even though it would take me 5 minutes).

I wanted to bring up something I noticed as well. When I replaced the motor in August, I replaced the AOS seals to the block. One thing I did NOT replace is the o-ring on the AOS cap. I looked through the intake runners last week and noticed an oily film on top of the AOS. All hose clamps are tight, so I doubt the leak is coming from there. However, is it possible that the seal on the AOS cap is leaking and if so, could that be the cause of the residue and possibly be enough of an unmetered air leak to throw off my AFRs?

I'm grasping at straws here but don't want to take off the intake again if I don't have to.

It's possible but I don't think it's the most likely explanation. The AOS is connected to the j-boot, so technically yes a leak there can be a vacuum leak, but at idle the j-boot doesn't see much vacuum. The AOS has an o-ring in the oil filler cap, another one in the clip-on cap with the hose connection, and another 2 in the block connections. In theory any of these could cause a leak but I think it would be small as long as the o-ring is present. Unscrewing the oil filler cap with the engine running can make it run noticeably rough, but that's a gigantic leak.

Small leaks can make a big difference if they're downstream of the throttle plate, where the vacuum is strong.

[NCGermerican]

So I've had a ton of stuff going on since I started this thread and haven't had time to test cold/warm AFR's without the O2 plugged in (even though it would take me 5 minutes).

I wanted to bring up something I noticed as well. When I replaced the motor in August, I replaced the AOS seals to the block. One thing I did NOT replace is the o-ring on the AOS cap. I looked through the intake runners last week and noticed an oily film on top of the AOS. All hose clamps are tight, so I doubt the leak is coming from there. However, is it possible that the seal on the AOS cap is leaking and if so, could that be the cause of the residue and possibly be enough of an unmetered air leak to throw off my AFRs?

I'm grasping at straws here but don't want to take off the intake again if I don't have to.

It's possible but I don't think it's the most likely explanation. The AOS is connected to the j-boot, so technically yes a leak there can be a vacuum leak, but at idle the j-boot doesn't see much vacuum. The AOS has an o-ring in the oil filler cap, another one in the clip-on cap with the hose connection, and another 2 in the block connections. In theory any of these could cause a leak but I think it would be small as long as the o-ring is present. Unscrewing the oil filler cap with the engine running can make it run noticeably rough, but that's a gigantic leak.

Small leaks can make a big difference if they're downstream of the throttle plate, where the vacuum is strong.

Dang it, John I'm looking for silver bullets! Especially since my Autool smoke tester seems to be.....smoked.