User 52121

*sigh* Just when I thought I finally turned the corner, I start to think I should have just left it the way it was and dealt with it... now I seem to have found a NEW problem.

Problem A: On warm and and hot starts, she fires up instantly, then stumbles around 400-500 rpm. Sometimes she stalls, other times, she stumbles for a bit, then kinda shakes it off and the idle jumps up to ~900 or so (where it should be.) From there she runs great. Shut it off and immediately attempt to restart and I get the same thing.

Problem B: I can invoke the low rpm stumble if I *very* quickly tap the throttle. Just enough to get the car to rev up to, say, 1500, then return to idle... she will dip down to ~500 or so for 2-3 seconds, then jump back up to 900. With a larger rev (enough to rev it up to, say, 2000) or when coming down to a stop from driving at speed, idle goes instantly and solidly to 900 with no funny business.

Problem C: I noticed this morning on the way into the office (I work from home 99% of the time, today is just special) that, when creeping in traffic, I started to get an odd hesitation. If I hold open the throttle juuuuuuust barely (so we're just off the "click" of the TPS switch) she stumbles and sputters and almost wants to stall. This is new, I never noticed this until today. I spent 2 hours in stop-and-go on my way downtown (which I've never really done in this car until today.)

I think all three above are related.

My inclination is that it's the TPS. However, the TPS *looks* recent (though I can't find a receipt or note from the PO on it.) I cleaned it with carb cleaner about a week ago and after that, the car started idling around 900 (prior to the cleaning it was idling at 1200.) The metal part of the sensor (where it bolts to the TB) is still a shiny new looking gold color, which makes me think it's been recently replaced.

However, my other thought is that it could be the AFM. Perhaps there is a dirty spot on the internal electrics right where it reads when just off idle.


Relevant items recently replaced:
ICV
O2 Sensor
Reference sensors
Vacuum lines (and I double-verified no leaks yesterday.)
Plugs
FPR and damper

Thoughts?

It's a good thing I like tinkering with cars...

Van

A low RPM like that is probably an indication of a rich fuel mixture. A lean mixture will cause a rise in RPMs.

If you have a FPR problem, it's possible that it won't reduce the fuel pressure enough at idle, causing a rich condition.

It could also be a TPS - off click, it might be giving the DME a signal that it's open more than it really is. This would trick the DME into going higher up a fuel map and providing more fuel - again, creating a rich condition.

My understanding is that the AFM is more of an indication of engine load - I don't think this reacts significantly to a small throttle blip like the way a TPS or vacuum signal does.

User 52121

Well it's a brand new fuel pressure regulator - so for now, I'll rule that out. I guess for now I'll go with TPS unless anyone has any better suggestions. Anybody have a KNOWN GOOD TPS they'd be willing to sell cheap so I can verify this is the issue?

Van

I know you said there are no vacuum leaks - but have you plumbed the FPR to the right vacuum port? E.g. before vs. after the throttle body...

User 52121

Vacuum lines are routed exactly as the Lindsey diagrams show (w/stock configuration and cycling valve.)

Line comes off the double-banjo on the intake manifold to a T. One side of T goes to FPR. Other side of T goes to a 2nd T - with one side of that T going to the damper, and the other side going to the bypass valve.

I hooked up a mityvac directly to the hose off the double banjo - and that whole mess (FPR, damper, and bypass valve) hold vacuum.

User 52121

Many thanks to Caster951 for the used TPS. Swapping that in did the trick! Car fires instantly when warm, goes RIGHT to idle. No more hesitation right off of closed throttle either.

I compared the two TPS's on an ohm meter (pins 2/3), and there was a pretty big difference between the two. The bad sensor went from 1k at closed throttle to 6.5k at WOT. The new one was around 500 at closed throttle and 3.8k at WOT.

quinnfiske

"I compared the two TPS's on an ohm meter (pins 2/3), and there was a pretty big difference between the two. The bad sensor went from 1k at closed throttle to 6.5k at WOT. The new one was around 500 at closed throttle and 3.8k at WOT."

You may have discovered something here. Maybe you had a TPS from another car (BMW or something) on there. It is possible that these gizmo's have different internal resistances for different applications. Just a guess, but something to keep in mind.

dillon410021

I will have to check my tps. I get kinda the same stumble as you and the revs drop low just like yours! which pin is #2 and which is #3 ??

User 52121

It's labeled on the TPS itself under the plug.