2.7l surging problem - BMW Forum

DIRTE30 · 08-08-2006, 09:27 PM

OK... So I've got the 1k dollar bargain mobile on the road but not without a hitch... albeit a small one... I hope! Starts and runs smoothly at idle but under load it surges something terrible. Strong, stumble, strong, stumble, etc... I found several problems that I had hoped would lead to the solution. First was that the CAT was broken and turned sideways and was blocking the exhaust. Straight pipe! Next was the intake hose was torn at the bottom in the accordion part where someone wasn't too nice while changing the filter! Duct tape while waiting for a new one. Still does it. I can get it to pull hard all the way through red line if I go WOT. That leads me to think that it's a closed loop sensor of some sort. Perhaps an 02 sesor or a TPS problem. I'm not terribly adept with this particular engine and its paculiarities so I thought I would fish around in here to see if someone may have run into this before I go testing all the sensors in the heat!

DIRTE30 · 08-08-2006, 09:29 PM

Oh... it's an '87 325 coupe. ETA motor. 200,000k on the odo.

Starmavin · 08-08-2006, 09:54 PM

Considering everything you just said, change the fuel filter. Next, check the TPS. Also Ohm check the AFM you may have a dead spot.

DIRTE30 · 08-08-2006, 10:10 PM

Sounds logical. It doesn't feel like a fuel issue, to consistant. The AFM thing sounds like it may be the ticket though. Is the TPS a conventional sweeping unit or is it a two switch type?

Starmavin · 08-08-2006, 11:26 PM

Switch. Check the PVC hose to the valve cover its common to have the break or crack. A faulty temp sensor can cause this as well. Another thing to check would be to unplug the cold start injector after starting, I have found that the thermo-time switch can kick on and off by it's self when bad.

DIRTE30 · 08-09-2006, 12:33 PM

PCV hose is good... I know this because when I took the throttle body off to clean it I took that hose off as well and you would not believe the crap that was stuffed in there! It took me a good half hour to clean it out. The coolant temp sensor and the cold start valve sound like roads to follow though! Thanks buddy!

PhatCat · 08-30-2006, 04:53 PM

Just thought I would throw in that I seem to be having the same issue. '86 325e, runs smooth and very well, but under a heavy-ish right foot, it feels like the trottle pulses, consistently. A couple of the guys I have spoken with think it might be the mass airflow meter(so they say!). I would be the first to admit that my knowledge of this car is still quite limited, but the explanation made sense. Apparently the gate inside the unit won't consistently stay open, and will flap a bit under load, hence the surging. Do I sound like I'm talking jibberish, or is this maybe the cause?

gotKRAUT · 08-30-2006, 05:46 PM

i have found this to be a common problem with both my Etas now, shorter rear diff and chip seams to cure it though, and full ignition tune up

PhatCat · 09-19-2006, 10:57 AM

I swapped out my AFM(I had seemingly the same issue, and it helped significantly). I did put in a pre-loved AFM though, I'm thinking a brand new one would run smooth as silk.
...still waiting for my friggin chip...

Rfitz25 · 09-19-2006, 03:30 PM

yeh I returned 2 bad rebuilt AFM's
got a new Bosch one, help smoothed things out. expensive
my TPS was bad and had surge/hunting at idle too, new tps fixed that
have to check the gap on throttle plate too , feeler gauge and adjusting screw.

read a thread about rebuilding AFM but its really hard. opened my old on up and I coud see the grooves worn in the sensor surface. If you can get the little arms to ride on a new surface im told it can last another 10 years.

PhatCat · 09-19-2006, 03:56 PM

Rfitz, I haven't taken my old one apart yet, but would it be possible(given enough access and room) to perhaps fill the worn surface with some sort of epoxy and polish it clean again?
The one I got second hand wasn't even rebuilt, but it runs smoother than the original off of my motor....it was cheap too, we have an auto graveyard here that is pretty much dedicated to German cars, so they have all kinds of stuff to pick through that is in pretty good shape (that, if in good condition, works just as well as something 5 times or more the price off the shelf, new.

Rfitz25 · 09-19-2006, 05:44 PM

I'm no expert just a budget garage wrencher.

If you can get a spare, maybe go for it. Its all delicate mechanical work. Just dont wreck the electronics.

I wouldnt open the one that works. just clean it up and lube it, bench test it with a DMM. I used some electronics parts cleaner and WD40 on my old one, and it seemed to help.

You can bench test the resistance across the arc from closed to WOT.
Ther is a big drop change in resistance at WOT that is supposed to be there..
Check out the manuals.

You can't fill the groove though its some kind of calibrated variable resistive surface material.
I think the resistance varies based on disatance from the pickups.

The surface is like some kinda dialectric material or something on a circut board with a bunch of traces on it and contacts at different positions along the arc. theres a copper arm that extends off the center post and two little ball bearing like ends slide on this surface.

The forum posting I saw said the guy carefully took all the hardware off the center post did something to this copper piece so it would be a bit shorter or longer. cant remember which. but still retain the same pressure on the surface it rides on. It then rode on a clean spot of this magic coating material, read a smoth resistance from closed to WOT and ran like a champ for years. It looks basically like a fancy 80's technology variable resistor.

I'm a software engineer so I explained it to one of our electrical engineers and he tried to explain how he thought it works to me. LOL.
I think its worth a try since a new one is $300-350 and the data is critical to the engine running correctly.
If it works you should write it up here. Seems everyone with an old AFM has trouble.

Good luck.

gotKRAUT · 09-20-2006, 12:17 AM

fitz, did you use the Bosch Rebuilt AFMs or the off brand rebuild? i was just curious because when i pull up the part it shows one rebuilt by bosch and another rebuilt by someone else i cant remember off the top of my head

Rfitz25 · 09-20-2006, 12:32 AM

local shop, inner auto sells em from some other supplier.
Ihate that shop now.
I called the said they had new roundels in stoick I drove down there on lunch break and they only had one. I was so pissed, I left w/o anything. Gona getem from bavauto.
I'm certain it was not Bosch.
They said they were gona stop carrying them.
Carl my mechanic "the bimmer doc" also0 said he has had bad luck with rebuilds and now only sells new Bosch AFM's
(of course $$$)
I'd trust a Bosh rebuild.

But dam that new unit looks clean.
If I could just get the rest of the engine bay to look and run as well.
....patience young skywalkler. one Bosch part at a time

PhatCat · 09-20-2006, 03:29 PM

I found an AFM for $215.48, not sure if it's rebuilt or not, it doesn't say. It's by 'Fuel Injection Corp.' Not sure if i would trust to order something like this though...hard to say. Here's the addy to see it. The Bosche one the same site sells is over $400

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