Consistant surging (hesitation?) under acceleration

[Woulf]

Hi all, I'm new to both this forum and the marque, but not to cars in general. Blah blah introduction. Here's the car:



And here's a picture of my wife and the car:



She hasn't been a great source of help. The problem is that when idling, if I listen real close at the tailpipe, I can hear the RPM drop ever so slightly every few seconds. When I accelerate, that's when the surging starts (though it's really more of a brief hesitation). Everything's cool at first, then it bogs for a brief second (not terribly mind you, but enough to notice), then it's right back to normal.

I've listened carefully for vac leaks around the engine, can't hear anything. I checked the air intake boot, nothing abnormal there other than an aftermarket "POP" filter. Connection is good to the meter as well.

The only other item of note is the smell of fuel after driving for a while. We first noticed it last night (I just purchased the car yesterday) which leads me to believe this may be an o2 sensor issue. My theory is that the car is running rich, the o2 snaps it back to lean for a moment, wash rinse repeat.

I've done some searching on here and mostly found everyone either saying vac leaks or AFM. Before I go and throw parts into a car I know little to nothing about, I was wondering if any of you kind folk had an opinion on the matter.

Thanks for looking!

[e30iwite]

Mine does that too. Don't know why it does it. I'm guessing that the ecu needs to be tuned or something to do with the ignition, timing, belt slipping???

Anybody here know how to fix this?

Love the red dress

[MTechnikII]

might be a feul pump issue, (I think that model has 2 fuel pumps, one in the tank and one outside )or even a fuel pump relay problem, unplug the O2 sensor see what hapens. but My vote goes to AFM.
Henry
PS: on a car that old when you smell gas, change all the feul hoses and feul filter.
when the pump is noisy , it's time for a new one, it was noisy on my 87 325es too but never got to change it, the car got totaled before the pump was gone.

[browntown]

Excellent first post. That should be a requirement around here.

[Woulf]

unplug the O2 sensor see what hapens.

My only concern with doing that is I don't know what the Motronic unit will default to. Though I suppose d/cing it only at idle would be safe. I'll give that a shot tonight after I go handle the registration.

PS: on a car that old when you smell gas, change all the feul hoses and feul filter.

I'd normally agree except that we only smelled it briefly after some in-town driving, not on the highway or when the car is simply parked.

But! This morning when I was warming the car up I noticed the fuel pump is somewhat loud. Is this normal for these cars? If not, I think I may have just discovered another great thing to check.

[jlevie]

Surging while driving could be a fuel system problem (dose the econometer drive towards zero when the car looses power?). But it could just as easily be from intake leaks, a bad AFM, flaky timing reference sensor, or other problems. It would be a help to know the year and model.

[Woulf]

Surging while driving could be a fuel system problem (dose the econometer drive towards zero when the car looses power?). But it could just as easily be from intake leaks, a bad AFM, flaky timing reference sensor, or other problems. It would be a help to know the year and model.

It's an 84 325e going on about 260k. I didn't notice the econometer doing anything funny when this happens, which leads me to believe there aren't any vacuum leaks. That thing IS vacuum based... Right? I did notice it not taking the traditional dive to the left during deceleration, however.

So I brought out the carb cleaner and started spraying anyway. No increase in idle, no leaks.

What's really strange about this is how static the frequency is. The stalling effect NEVER changes, no matter how far on the throttle I get. It always bogs out with the exact same pulse.

BRB, I'm pulling the AFM to see if it's jammed or something. While I'm thinking about it, what port on that little connection under the hood would I use to test the oxygen sensor? Does anyone know?

[Woulf]

As I figured, he AFM isn't stuck. Actually, it's rather clean. And seems to function as normally as can be expected of something that determines airflow by measuring how far open a friggin vane is. Meh, hate the buggers, I prefer MAF.

Sooooooo, we're down to the fuel system or the o2 sensor... Although I have another theory! Test test test...

[Woulf]

And it's not the fuel pump relays either. No clicky clicky correlating to RPM variances. So at some point tomorrow afternoon I'll pull the Motronic and check the o2 sensor voltage. More data, more theories, more testing. All in the hopes I don't have to get under yet another car and work on the fuel system. I really hate getting rust all over my face, and gasoline does not smell good after having it dripped on you for 2 hours straight.

[hotballs]

Your car will run fine without an oxygen sensor. I never ran one. The slight blip in RPMs when you listen real close could be a leaky injector.

The hesitation, which doesn't sound too terrible, might just be the way the car is, being 25 years old and all. I don't think there is really that great of throttle response on a stock eta with a barn door style AFM.

All in all though, etas can smell like fuel and they have more going on than the 325i's. They have 4 sensors in the t-stat housing instead of 2. Maybe one of those is all outta whack. The "cold start valve" could be open messing things up. The charcoal canister could be clogged up causing vacuum leak style related problems.

[riceh8r]

pump is basically noisy...just so you know.

[7pilot]

I would remove the rear seat, remove the access hatch for the lift pump.
Then jump the pump relay and attempt to detect lift pump function by sound or vibration.
If the lift pump is AWOL, then the main pump will become noisy and fail in short order. (Perhaps that is why it is loud).
If the hesitation gets worse with decreasing fuel level, that is a sign that your lift pump is dead.
Other possible culprits could be a bad idle switch in the TPS, or static timing that is a tooth retarded.


m

[Kershaw]

could definitely be injector related if you are saying it is ALWAYS there.

if it were the o2 sensor the issue would not be present in the first few minutes of operation since it is basically bypassed while it heats up.

[Woulf]

The charcoal canister could be clogged up causing vacuum leak style related problems.

Fascinating. Wouldn't that also cause me to have trouble refueling the vehicle due to the tank not depressurizing during fill-up? Because that's something I was just going to deal with eventually. The only reason I mention it is I had an 86.5 Supra with a 1JZ and no charcoal canister, and the pump always shut off while fueling.

I'm going to check the pre-pump after work to make sure it's functioning. As riceh8r mentioned, the pumps seem like they're constanly making noise on these cars, but it'll change pitch once I remove power to the pre-pump if it's indeed functional.

Still looking at that darn o2 sensor, though. It's the fact that it's a consistant, persistant pattern that's driving me nutters. I swear it's something in either fuel delivery or fuel management that's supposed to be variable that's stuck in a pattern.

[Woulf]

Went outside on my 15 minute break and took the vac hose off the canister and corked it. Gave it a little gas, same problem. Originally though this might rule out the charcoal canister, realized I should have left the line disconnected and tried again. D'oh...

Note to self: Vacuum lines taste bad.

Lunchtime Edit!: Went out again, this time tested with my thumb capping the line, then without my thumb capping the line. No change either way. So yes, now I can conclude that the charcoal canister is not the issue. Any result's a result.

What I DID notice, however, is that I had to warm the car up a little to get it to start the pulsing. Also, the engine doesn't increase it's idling speed when cold, it just parks at about 900 immediately upon startup. My brain is running wild with potential things to check, including any and all of the following:

Oxygen sensor
Coolant temp sensor(s?)
Idle Speed Control Valve (or whatever BMW calls it)
Fuel Pump(s)

The question is, where to start? I can check the voltage to the temp sensors and the o2 at the Motronic unit and then at each sensor if I don't like the results I get there. Off to test the fuel pump by pulling fuses.