Misfiring problem at high revs (only while driving)?

[Moxie]

Thought I'd repost this from Bimmerforums since I'm not getting much action over there:

Hey guys,

Back for more in this epic saga of getting my E30 out of the garage for more than 5 minutes. So I recently did quite a bit of work on the E30. I did the whole timing belt kit, replaced the entire right side of the rear end, and replaced all the struts and springs. Got her on the ground this past Sunday, and other than a bit of air in the cooling system, all was well.

Until today... I was merging on the interstate and all of a sudden the engine starts cutting out like I hit the rev limiter or something, but I was only at about 2k RPM's. Downshifted from 5th to 4th, same story. Pulled over and let her cool off, and when I started her back up she started sputtering again. After a little more cooling off, I finally got it to a gas station. Everything looked fine. Car started right up with no problem. Got out on the road again and it was still misfiring at around 2k RPM's.

When I got it back to the garage, I let it sit there idling and revved it up (via the throttle body), and it revved fine all the way up to redline.

This is the strangest thing I think I've ever seen out of this car. I'd suspect the fuel filter was clogged, but wouldn't it still be misfiring revving in neutral? I also thought maybe it was bad gas or a bad cat, but I can't figure out why it would only do it while moving???

Also, I know people like to blame bad gas for a lot of things, but the car was sitting for almost 5 months, and I didn't even think about putting in fuel stabilizer until I was taking her of the jack stands (whoops). I did put some Heet in there today, so we'll see if that does anything.

To summarize:
1. Car starts misfiring in a really jerky way at 2k RPM's ONLY when moving.
2. Idles fine.
3. Half of the gas was about 5 months old.

Ideas? Thoughts? Comments? All are greatly appreciated!

[e30construction]

have u checked the ignition parts? plugs,wires,dizzy,rotor,coil?

[jlevie]

I get irritated at folks that post the same question in multiple forums at the same. Start by posting in one forum and look at the responses before posting in another...

5 month old gas isn't great, but it won't cause that. What you are seeing is load sensitive misfires. That can can be from ignition problems, fuel delivery problems, a problem with the AFM, or even a bad CPS.

[Moxie]

have u checked the ignition parts? plugs,wires,dizzy,rotor,coil?

I've replaced (within the last 6-8 months) the wires, plugs, and rotor. That might not eliminate them as potential issues though.

I get irritated at folks that post the same question in multiple forums at the same. Start by posting in one forum and look at the responses before posting in another...

Sorry, man. There's considerable overlap between members in forums, I know, but you never who's going to see it, and how long you'll have to wait for responses in one versus another.

I already read your post on Bimmerforums though. Thanks for the info.

I think based on the feedback I'm getting, I'm going to go ahead and take it to the shop down the street for diagnosis. I don't have the appropriate tools for doing a lot of the testing that needs to be done (i.e., smoke tests, etc.) and I certainly can't afford to be buying several $100+ parts to just start swapping out. When I get the diagnostic done I'll followup so others can have the info.

Thanks, again!

[panmarcum]

2 weeks ago my car was doing the same thing in the below 2000 rpm band. So i went to my buddy,s shop and put a pressure gauge on the fuel line and all I had for fuel pressure was in the lower 30 psi. Since i had helped him the previous week to change both sending units of which one had a working fuel pump on it from a 2006 3.0 X3 we die grinded the plastic off of it and partially modified the one on my 90 325is fuel pump and it has been working fine ever since. But i did splurge on a new fuel filter $8.00.

[e30construction]

yea i just got my fuel filter today..plan to swap the plugs,wires,dizzy,and rotor on friday as well as the fuel filter...anybody have any experience with stp full fuel system gas treatment? im planning to add it after i change everything out..or wld it be a better idea to add it before changing the filter? maybe to put anything that breaks up into the old filter then change it?

[jlevie]

yea i just got my fuel filter today..plan to swap the plugs,wires,dizzy,and rotor on friday as well as the fuel filter...anybody have any experience with stp full fuel system gas treatment? im planning to add it after i change everything out..or wld it be a better idea to add it before changing the filter? maybe to put anything that breaks up into the old filter then change it?

And now you did the same thing on this site by asking in the General forum about fuel treatment and then in this question...

Have some patience. Ask a question in an appropriate forum, then see what responses you get over SEVERAL days before posing the question elsewhere. If you need an answer faster than that, take the car to shop and pay for a diagnosis by a pro!

[DmcL]

old/worn ignition system parts can cause misfires, generally worsening as RPM gets higher (if you can rev it higher that is). AFM can also cause problems and a bad or chafed CPS sensor/sensor wire can cause no starts, loss of fuel/spark either all together or intermittently.

check those things first and foremost. my first stabs would be look at plugs/wires and condition of distributor and rotor arm. after looking at that check out the CPS sensor, most importantly follow the wire from it and make sure the water pump pulley hasnt started chafing the wire. if i had to bet id say its one of those two things, either ignition or CPS.

[Bmwb]

this problem just happened to me today ... when i was driveing back home from school , i tryed to see if it was fine when it cools down but was still jerking , just changed my spark plugs/ wires / cleaned my ICV ,and my throttle body including the MAF which was filled with sludge , but i got a stroker so idk if it would be the same but yea , but cant u always just get the codes from the computerr " key ignition on position dont turn on the car press the pedal to the ground 5 times in about 3-5 sec and watch your check engine light flash and your good from there tell us how much flashes u got etc.

[vert_this]

Ive had the exact problem on more than one m20 and on two occasions it was the CPS. It could be other ignition items as others have said however you make sure to ck the cps at some point.

[Moxie]

The CPS had come unsecured and was rubbing on an accessory belt. Didn't figure it out until after replacing almost the entire ignition system though. On the bright side, the engine has never run this smooth in the 8 months I've had the car!