Help me diagnose an intermittent bucking/CEL problem

[hasback]

just replaced the cps with a new bmw one that ohmed out perfect, and im still having these issues. whats my next step?

[dnguyen1963]

smoke test is your best friend.

[FLG]

You need to ensure the dme is not losing power (read my previous post) also inspect all connectors (especially c101) for corrosion.

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[TC Baur MD]

A brand new bosch ICV helped with my rough idle and newly rebuilt E36 fuel injectors helped correct my hard starts.

[kwseattle]

You need to ensure the dme is not losing power (read my previous post) also inspect all connectors (especially c101) for corrosion.

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I'm still not looking forward to figuring this one out...

I'm going to try a spare AFM that a friend has, and take the no-name chip out of my ECU and see if that helps it stop bucking/running rich. I'm wondering if it's just throwing more fuel without any real reason. All I know is that the previous owner called it an "AutoWerks Performance Chip." I haven't gotten to installing my CPS yet, school is getting in the way again.

A brand new bosch ICV helped with my rough idle and newly rebuilt E36 fuel injectors helped correct my hard starts.

Can confirm E36 fuel injectors (mostly) fixed my hard starting issues and idle.


I haven't taken care of my vacuum leaks yet, I ordered literally every sized vacuum hose Pelican had in the E30 catalog (including the breather hose and intake boot) to take care of that problem. Most of them are old with frayed ends, and a couple broke apart. Also ordered a fuel filter, throttle cable, and a couple other things.

[kwseattle]

Update: vacuum leak is fixed, but the car still struggles to hold an idle in the morning, so I have to hold it at like 2k RPM for ~30 seconds while it sputters until it will steady itself.

I've noticed that when I let my foot off of the gas to coast, it jerks back and forth slightly, and it also doesn't seem to be cutting fuel - the MPG gauge only goes slowly up to 40.

Also, it seems that engine revs take quite a while to fall down when taking my foot off of the gas. It falls really slowly to idle.

Now it sounds like either my TPS is bad or misaligned, even though it tested good. I'm getting really weirded out by all of these problems... I think I'm going to try another ECU and maybe an AFM to see if that helps.

The check engine light still keeps coming on intermittently, and then going out.

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[hasback]

im waiting as well for a new ecu to see if that fixes my problem. because i have tried everything above and have had no luck.

[jlevie]

Update: vacuum leak is fixed, but the car still struggles to hold an idle in the morning, so I have to hold it at like 2k RPM for ~30 seconds while it sputters until it will steady itself.

I've noticed that when I let my foot off of the gas to coast, it jerks back and forth slightly, and it also doesn't seem to be cutting fuel - the MPG gauge only goes slowly up to 40.

Both of those could be caused by intake leaks. Have a smoke test run.

[hasback]

ecu made things worse. it instantly started making the car miss from the start. the other one was better but then later went into the misfiring. still have unresolved issues and ive ran out of options.

[kwseattle]

That's weird that the replacement ECU made it worse... I've heard of that solving a lot of problems.


In other news, my original problem that I set out to solve is still manifesting itself almost daily (completely random bucking, happens usually when accelerating and sometimes it'll stumble when idling), except now I have this other vacuum leaky crap to deal with. I hate to say it but I'm almost starting to regret buying a 26 year old BMW. I kind of wish I had gotten an E36 M3 instead, though I'm sure that'd be more expensive.

I think I'm going to end up bringing it in to a local Euro shop up here, there's one that practically specializes in old BMWs so they should be able to figure out what's up.

[jlevie]

That's weird that the replacement ECU made it worse... I've heard of that solving a lot of problems.

Actually that is not so weird. The DME will learn a fuel trim that compensates for problems. Disconnecting the battery/DME will clear those learned adaptations and can result in the engine running worse until the DME adapts.

[kwseattle]

Finally got around to replacing the CPS, running FLAWLESS.

Idles as good as it gets, no missing or bucking, check engine light is gone, throttle response is back.

The wire shielding was pulling up out of the sensor and there was a little tiny severed wire. That'll do it.

MAF + 19 lb injector time? :D

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[kwseattle]

Update (new post because I think it might be helpful):

So I just installed a Miller MAF conversion with a set of Bosch 19 lb/hr injectors (with the appropriate chip) that my buddy had laying around for sale.

ALL of my engine problems are gone. It starts immediately and settles to idle quickly. My idle is also now perfectly steady at ~800 and it makes POWER. It sounds so much better and is quite a bit faster. I think I'm addicted, now I want a set of cams. :D

Before the MAF and after the CPS replacement it wouldn't engage deceleration mode (cutting fuel when off throttle and coasting in gear) about half of the time resulting in jerkiness when slowing down. When first starting the car it also would tend to bounce the idle up and down until I had taken off and gotten above ~3k rpm.

New life has been breathed into my old, aching M20.