Stumble/Brief Hesitation 3500-4000rpm y0!1!11!1

[DmcL]

ok so this is really starting to annoy me.. have tried a few things but am none the wiser as to the cause.

basically the car runs perfect and occasionally will hesitate for a second between 3500-4000rpm then clear and be fine again. doesnt happen every time and seems to happen most in 2nd or 3rd tho has happened in 1st and possibly 4th.. also only seems to happen over 50% throttle. no CEL or anything.. MPG is averaging about 17 tho the cars chipped and running a 4.10 diff (w/moderate lead feet lol).

tons of stuff has been replaced last year such as entire ignition system from coil to plugs, fuel pump (filter is old, have one to go in tho) all sensors are new apart from AFM, ICV and CPS tho CPS was changed for a good used one last year. AFM has been opened and cleaned (inc. air temp sensor), carbon tracks checked for wear and i even tried adjusting the toothed wheel a couple clicks richer/leaner to no avail. also removed the home brew sealed intake and air feed laving only an open mushroom filter, again no effect. inlet manifold gasket was done last year, no splits in intake elbow, no major vac leaks anywhere, one small leak where the ICV joins the inlet manifold but very small. does it regardless of if im on a stock chip or not, havent tried another 173 (im not using the one that came in the car atm) tho i doubt thats the cause.. CTS, TPS also replaced.

im also wondering if it may be something with the ignition coil tho i dont think it is as it was new last year. im running an aftermarket coil. a Lumenition MS4 coil which requires/came with a 1.6 ohm ballast resistor. wouldnt affect spark timing would it? had no problems with it previously.. apart from that my only other thought is the fuel filter or maybe the FPR is starting to go? beyond that im going to be seriously stumped..

[DmcL]

also since im running an uprated coil can i open up the gap on the plugs a bit and how much by? plugs are NGK copper cores. half thinking if i can gap them a little wider i can try that and worst case rule anything relating to plug gaps out.

[jlevie]

Below 4500 rpm the AFM and O2 sensor are still in the loop. Above that the DME runs open loop if it sees the WOT switch in the TPS. The useful life of the O2 sensor is not more than 100k, so if that applies you need to replace the sensor. Visual examination of the AFM says, pretty much nothing. You have to electrically check the curve of the AFM as a function of vane position to know whether the AFM is good.

There is a resonance point roughly where you are seeing problems that can cause a weak or failing harmonic balancer to wobble, which can affect timing data from the CPS.

[DmcL]

car doesnt have O2. think its a US model thing.. dont think any pre91-92 ish cars had them here. car ran flawless with the same setup so unless the ECU i have in now is partially damaged or something has started to develop (problem hasnt changed/worsened in about 1-2 months).

what balancer are u talking about? there is no balancer on the propshaft unless ur talking about the pulley on the front of the engine or the associated toothed wheel? also what are the chances the new-old CPS is starting to fail at a certain point? any way of testing that? also the AFM? i have an automotive multimeter but dunno what values id be looking for when i measure across the different terminals or move the flap in the AFM.

[884door]

Below 4500 rpm the AFM and O2 sensor are still in the loop. Above that the DME runs open loop if it sees the WOT switch in the TPS. The useful life of the O2 sensor is not more than 100k, so if that applies you need to replace the sensor. Visual examination of the AFM says, pretty much nothing. You have to electrically check the curve of the AFM as a function of vane position to know whether the AFM is good.

There is a resonance point roughly where you are seeing problems that can cause a weak or failing harmonic balancer to wobble, which can affect timing data from the CPS.

I'm having a similar problem as him along with a check engine light, car is an '88 325e, the CEL blinks twice on startup which indicates o2 sensor malfunction, does this sound like a possible result of the o2 failing? It seems ignition related to me.

[jlevie]

I'm having a similar problem as him along with a check engine light, car is an '88 325e, the CEL blinks twice on startup which indicates o2 sensor malfunction, does this sound like a possible result of the o2 failing? It seems ignition related to me.

How many miles on the O2 sensor. It is a scheduled maintenance item with a useful life of not more than 100k. If that applies, replace the sensor and go from there.

[KillaCam]

I had this happen. I had a rotted fuel line sucking in air

[884door]

How many miles on the O2 sensor. It is a scheduled maintenance item with a useful life of not more than 100k. If that applies, replace the sensor and go from there.

o2 sensor mileage is unknown. I replaced my main relay and cleared the CEL and after about 25 attempts and 30min. of driving it hasn't occured again and the CEL hasn't came back, I have no idea how a bad relay translates to o2 sensor CEL code but I hope that fixed it. Oh, also my issue was occurring right at 3000rpm pretty consistently.

[DmcL]

I had this happen. I had a rotted fuel line sucking in air

i did notice one of the lines into the fuel filter is a little cracked.. no change in the problem before/after a new filter tho. its like a stutter that comes on just after 3500rpm and peaks just shy of 4000rpm where performance drops then when it hits 4000rpm it clears and the power kicks back in. doesnt do this all the time tho, sometimes if accelerating from lower down with foot on the floor it will rev right on past the problem area like normal.

[morerevsm3]

do you have afm bolted steel to steel (no rubber mounts)< I was told earthing the body of the afm can cause this, I have the same problem on my race car, and will try isolating afm next race

[lateracer]

Sounds like either an electrical or fuel starvation issue. A fuel pressure gauge set up so that you can take readings from the rail while the problem occurs will address that question. My vote is to start there as it's an easy thing to rule out.

Although the conditions of your problem sound like they aren't a perfect match, I've found electrical connections that came loose at this exact RPM range in both of my E30s. Something about that RPM range seems to shake things at just the perfect resonance to make loose connections fail and then work again when moving slowly -or faster in your case. Losing the top 2K is a REAL bummer during a road race. Symptoms I noticed from this were an inconsistent RPM cut off (sometimes 3.5k, sometimes 4) that became progressively worse.

[jlevie]

A fuel starvation issue will get worse as rpm and engine load increases, which apparently isn't what is happening here.

[joey30]

I'm bumping this old thread because I want to know if the OP ever figured out what the problem was.

I'm having the exact same problem with my car now. It only happens when the car is under load though. It cannot be recreated just sitting in the garage and revving the engine.