Sustained low rpm driving causing a bad hesitation...then clears up and is fine?

[strang3majik]

Sorry for the novel of a title...but....

On my 87 eta, if I'm driving around town at low rpms (sub 1800) for a sustained period of time, then go to accelerate, the car falls flat on its face to the point it almost feels like its going to shut off, dropping in rpm and everything, bucking, etc...

then it'll suddenly recover and take off like a raped ape and be fine til I drive at low rpms for a while again.


Any ideas where to start? Doesn't smell rich or anything when it does it, just falls flat on its face (its been bad enough before it popped and carried on)...then out of nowhere clears itself out and is off like a shot.


Car starts and idles great (only with the occasional idle fluctuation), cold/hot, doesn't matter, starts at the first flick of the key. Runs great otherwise as long as I keep my foot in it some. Just easy driving for an extended period transitioning to acceleration.

Also if its starts doing this and I come to an idle, it idles real rough and more or less runs like crap til I go full throttle to clear it out again.


Any help would be appreciated! I'm thinking MAF, but want to get some incite before I go replacing stuff.

[jlevie]

I'm thinking intake leaks. Have a smoke test run to find out.

[CorvallisBMW]

Could be intake leaks, but there are some other possibilities as well:

-partially plugged fuel filter
-sticking fuel pressure regulator
-sticking AFM flap
-dirty injector(s)
-fouled plug(s)

Do the checks first (free!) then start replacing parts ($)

[strang3majik]

Plugs, cap, rotor, and fuel filter are only about a month old...so they check out.

But I'll check out that other stuff then...like I said, I think the AFM is taking a crap. Somebody has modified it at some point so I think thats part of it.

I've done a fp check on it recently and it seemed within spec and responsive to throttle blipping, any other way really to check the fpr?

And as far as injectors...thats a possibility I would imagine. It does have in excess of 200ish k miles.


I also have a new transfer pump I'm throwing in it just for that warm fuzzy feeling (I'm driving it down to FL for Thanksgiving...lol)...and I can hear the main pump whine every now and then so I'm not sure the transfer pump is even operable.


I'll check for vac leaks too...I know of one, but its been there (hose going to the valve cover)

[paperplane94]

A bad Fuel pressure regulator would also give you those symptoms of warm engine hesitation.

[strang3majik]

Well I haven't driven it for a prolonged period, but did go beating around in it after installing the new transfer pump and mother of god what a difference.

It has a lot of new found power that wasn't there before and even spins instead of just chirping second. Of course its a one wheel peel, but still a large improvment...haha.



So not saying its solved yet, but that obviously was a problem.


But...to add to the previous and something that is still happening...only occasionally it'll hit 4k rpms like a brick wall, let off and punch it again...its fine. Thats only intermittent though, no rhyme or reason to it.



It did that once during my drive testing it out with the new pump...did it every now and then before as well.

[dnguyen1963]

A brief fuel starvation can cause your problem. Was your gas tank near empty when you experienced the symptoms? I would tee in a pressure gauge and monitor the pressure while driving.

[nny403]

A brief fuel starvation can cause your problem. Was your gas tank near empty when you experienced the symptoms? I would tee in a pressure gauge and monitor the pressure while driving.

Jumping in on this thread, I have the exact same symptoms. Is on your 325 the issue coming up more or less randomly? Or after the car has been driven to full operating temperature then shut off then started back up?

[strang3majik]

NNY403...its more or less random. The hesitation is only after driving at low rpms though.

And I'll be honest, after the new transfer pump, it hasn't done it yet, but I've only probably put 20 miles on it so far, so I'll keep ya'll posted on that.


But the brick wall/breaking up at 4k is still there, and is also random. It does seem like fuel starvation, but fuel pressures were in check when I checked them a month ago. But maybe the old dying transfer pump overstressed the main pump and now its dying.


I'm going to check the distributor cap again...make sure it didn't crack again....just looked at it today and it isn't sitting entirely flush :/

[nny403]

NNY403...its more or less random. The hesitation is only after driving at low rpms though.

And I'll be honest, after the new transfer pump, it hasn't done it yet, but I've only probably put 20 miles on it so far, so I'll keep ya'll posted on that.


But the brick wall/breaking up at 4k is still there, and is also random. It does seem like fuel starvation, but fuel pressures were in check when I checked them a month ago. But maybe the old dying transfer pump overstressed the main pump and now its dying.


I'm going to check the distributor cap again...make sure it didn't crack again....just looked at it today and it isn't sitting entirely flush :/

The same symptoms are happens with mine as well, but prior experience points to pressure regulator issues. Happened a lot with Datsuns. BMWs seem to be less clear in their diagnostic procedures. Replace this part, cry, and then replace another part. Not the most viable method.

[85e325]

Im having a similar problem with my 85 M20b27. When Im cruising around between 1500 and 2000 rpm then accelerate it will drop about 400 rpm and hesitate for about 5 sec and then smooth out. Here's what Ive done to it so far...

cap, rotor, plugs and wires
timing belt
air and fuel filters
cleaned MAF and injectors

I do have a moderate exhaust leak right behind the muffler, would something like that cause it? Its not effecting my gas milage or the driveability of the car, its just REALLY annoying.

[strang3majik]

Welp...replaced the cap and rotor again for the hell of it since they were under warranty and checked the wires while I was at it an they were arcing like mofos.

Replaced them as well with another used set I had...no more high rpm breakup.

Like I said this was only after a few mile drive. Ill be driving it all around tomorrow so ill keep everyone posted.

[nny403]

Welp...replaced the cap and rotor again for the hell of it since they were under warranty and checked the wires while I was at it an they were arcing like mofos.

Replaced them as well with another used set I had...no more high rpm breakup.

Like I said this was only after a few mile drive. Ill be driving it all around tomorrow so ill keep everyone posted.

Please do, I'll report in on the pressure regulator if I can get the symptoms to duplicate and see if we can't tidy up the fixes in a single thread. Searching on this forum is a bit limited.

[Chriskbmx]

Welp...replaced the cap and rotor again for the hell of it since they were under warranty and checked the wires while I was at it an they were arcing like mofos.

Replaced them as well with another used set I had...no more high rpm breakup.

Like I said this was only after a few mile drive. Ill be driving it all around tomorrow so ill keep everyone posted.

are you using a boush cap or a generic one? im just curious

[strang3majik]

are you using a boush cap or a generic one? im just curious

Its a Duralast but says BOSCH and Made in Italy right on it. All Duralast ignition stuff is made by Bosch anyway.


And the hesitation is still there, but much less apparent. It hasn't done the high rpm breakup since the wire and cap/rotor swap though.


I wouldn't doubt its a fuel delivery problem for the hesitation...being the transfer pump def helped. I'm thinking the inline pump...I kinda want to replace it anyhow, being...I'm making a 1200 mile trip on a 1987 fuel pump on a car with in excess of 200k miles...haha.


Anyone know of any cheaper routes than the stock Bosch unit that are just as good? I just have a hard time spending around $200 for a nothing special inline pump just because it says Bosch on it. All auto parts stores have a similar price.