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E36 M3 (1992-1999) {Euro - S50 B32 321hp @ 7400 rpm} {U.S. - S52 B32 240 hp @ 6000 rpm}
Total Produced: 71,212 - Years Produced: 1992 to 1999


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Old Sun, Jun-24-2018, 07:30:23 PM   #1
bmos
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Default Lean at tip-in

I have been noticing what seems to be an open-loop AFR issue with my 206k mile s52 98 'vert for a year or two. It causes detonation from 2.8-4.2k RPMs above half throttle, dramatic bogging after cold start if I try to engage the clutch without first idling for 30 seconds or so, and a very occasional backfire when I blip the throttle from low RPMs when downshifting with a warm engine.

When I first noticed this behavior, I was running the Turner Motorsports Shark Injector tune which I promptly removed assuming that the timing was too aggressive.

I exclusively run TOP TIER 93 octane and have experimented with octane booster to validate that what I'm hearing is detonation. It takes ~98 octane to eliminate any audible detonation.

The powertrain is now entirely stock other than a short shifter, an s54 z3 radiator, Stewarts high-performance water pump, aluminum thermostat housing, and ECS silicone hoses. I have no engine codes and haven't since replacing the o2 sensors about 25k ago.

Here is a dyno chart showing the issue. It seems that quickly getting on the gas isn't triggering the appropriate level of fuel enrichment although it does somewhat catch up later in the powerband:

The AFR graph on this chart was generated with a post-cat clip-on wideband gauge so it probably is not quite so lean although it should be very close.

Any ideas/help would be much appreciated. I have a set of knock sensors I am considering installing but I am doubtful that is the issue as there hasn't been a code thrown. As the car pulls hard to redline I also believe the cat to be flowing freely.

What I've tried so far:
new fuel pump (VDO) and relay
new fuel filter (MANN) (~10k ago)
new MAF (Bosch)
new OE fuel pressure regulator
full intake reseal (duct, IAC hoses, manifold gaskets, all vacuum lines)
oem underbody fuel lines
fuel injectors (brand new Bosch units)
cleaned IAC (it seems to move freely without sticking)
coolant temp sensor
intake air temp sensor
lots more have been replaced but these seem the most relevant.

There are no vacuum leaks; I have tested for them very comprehensively with a smoke tester.

EDIT: I should note that the coolant level acts really strange. When completely cold it is too low. When I take the cap off the expansion tank, the level will rise until the bottle is overfilled. I'm not sure if this is an issue with the s54 z3 radiator or the coolant bottle but I have seen it mentioned elsewhere by people with this setup.

Last edited by bmos; Thu, Jul-12-2018 at 12:12:53 AM.
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Old Mon, Jun-25-2018, 03:31:28 AM   #2
NolliM3
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Default Re: Lean at tip-in

You haven't mentioned changing the fuel filter...that's my suggestion; however after a second reading I see you did (Mann).
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Last edited by NolliM3; Mon, Jun-25-2018 at 03:35:37 AM.
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Old Mon, Jun-25-2018, 04:45:14 PM   #3
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Default Re: Lean at tip-in

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Originally Posted by NolliM3 View Post
You haven't mentioned changing the fuel filter...that's my suggestion; however after a second reading I see you did (Mann).
It was before I replaced my underbody fuel lines that rusted through so it might not be a bad idea to re-do it. I might have done it again with my fuel pressure regulator but I can't quite remember. I have too many cars...

Just looked through my receipts. Looks like I initially replaced the fuel filter with a Febi filter in August of 2016.
I have not replaced it since then. So I'm going to re-do that with a MANN filter since they're the OE supplier and the fuel line must have contained a little rust if it started leaking.

Last edited by bmos; Mon, Jun-25-2018 at 10:22:24 PM.
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Old Mon, Jun-25-2018, 08:45:29 PM   #4
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Default Re: Lean at tip-in

Could it be the throttle position sensor?
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Old Mon, Jun-25-2018, 10:12:31 PM   #5
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Default Re: Lean at tip-in

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Could it be the throttle position sensor?
I used a scan tool to monitor the TPS signal and moved the throttle from closed to open slowly and saw no spikes, gaps, or other noise in the signal. It appears to be working perfectly.
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Old Fri, Jul-27-2018, 07:24:29 PM   #6
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Default Re: Lean at tip-in

No change with the new fuel filter. Definitely is dependent on ambient temps.
Might get a CAI that relocates filter into the bumper to try and lower intake temps but the stock intake system should be totally fine by all rights.
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Last edited by bmos; Tue, Jul-31-2018 at 12:23:41 PM.
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Old Fri, Aug-03-2018, 03:46:12 AM   #7
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Default Re: Lean at tip-in

sticky fuel pressure regulator?
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Old Fri, Aug-03-2018, 12:34:28 PM   #8
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Default Re: Lean at tip-in

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sticky fuel pressure regulator?
I replaced it after this started happening. No change
Thanks for the suggestion!
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Old Fri, Aug-03-2018, 07:27:31 PM   #9
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Default Re: Lean at tip-in

TPS would usually get me vote also. You could have a corroded wiring issue causing intermittent issues. The insulation is known to deteriorate and start shorting. You may want to start pulling back the insulation at the connectors (like TPS) and see if everything is in good shape. This would be where I’d be looking at 200K and all vacuum suspicions cleared
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Old Sat, Aug-04-2018, 03:01:40 AM   #10
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Default Re: Lean at tip-in

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Originally Posted by englishtom1596q View Post
TPS would usually get me vote also. You could have a corroded wiring issue causing intermittent issues. The insulation is known to deteriorate and start shorting. You may want to start pulling back the insulation at the connectors (like TPS) and see if everything is in good shape. This would be where Id be looking at 200K and all vacuum suspicions cleared
I'll do that. Thanks!
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Discussing The definitive guide to self diagnosing your ABS/ASC light. in the E36 M3 (1992-1999) Forum - {Euro - S50 B32 321hp @ 7400 rpm} {U.S. - S52 B32 240 hp @ 6000 rpm}
Total Produced: 71,212 - Years Produced: 1992 to 1999 at BMW M3 Forum.com (E30 M3 | E36 M3 | E46 M3 | E92 M3 | F80/X)