M20 320i Engine misfires backfires and lacks power.

[Snowey]

I’ve changed the gearbox, sump gasket fuel tank and metal fuel pipes. Now the engine won’t run properly.
It spits back when starting, (could it be head gasket) hunts, misfires on the road and lacks power.
It was ok before so I’ve guessing it’s a fuel problem. I’ve removed the filter from the bottom of the pump as it was full of rust from the old tank. I’ll check fuel pressure when the tool arrives but I’m clutching at straws and any help where else to look would be much appreciated.
Thanks David

[Speedtouch]

First off, what age is the car - i.e., is it pre-facelift with the chome bumpers and L-Jetronic or facelift with Motronic?

I would start by checking basics such as the engine-to-body earth strap and compression readings (should all be roughly equal across all cylinders).

Misfiring/backfiring is usually a sign of either ignition timing issues or possibly stale fuel, perhaps? Probably also worth changing the fuel filter, if you haven't already done so...

[Snowey]

Hi Maurice Thanks for the reply. It’s a 1988 cabrio with the 63litre tank and motronic. I Don’t think it’s the ignition system as it was ok before I did the work, although it was getting harder to start which I put down to a very rusty fuel tank blocking the filters and reducing fuel pressure. The filter on the bottom of the fuel pump was covered in rust (cheap Chinese pump I think) but the in line filter looked ok when I cut it open after replacing it. I’ll try the fuel pressures next week and get back to you. Does anyone in the forum have a recommendation of fuel pump manufacturer to avoid another Chinese replica?
Thanks

[Speedtouch]

Speak to Cotswold BMW about it, who sell genuine parts on this forum.

viewforum.php?f=39

[Snowey]

Ok so I’ve got my fuel pressure gauge and as far as I can tell it’s just abut normal 2.5 bar with the pressure regulator pipe and rising to 3.0 with the pipe off.
What is strange is when I block the return pipe the pressure rises to around 6 bar as you’d expect however the engine runs normally, nice idle and rev’s up as it should. What could cause this I’m guessing a weak mixture perhaps an air leak, manifold gasket perhaps? Any advice is always appreciated.

[Snowey]

Checked the compressions today all around 8 bar +or - 1/4 bar. Had a look at the air flow meter carbon brush all looks ok. Checked for intake leaks certainly nothing major enough to cause this much of a problem.
When I open the throttle slowly there is a massive flat spot at about 1500 rpm and spits back sometimes, if I block the fuel return it runs ok but rich.

[Speedtouch]

How does it behave at idle, and which ECU do you have, e.g., Bosch 0 261 200 172?

Some Motronic ECUs had additional diodes in the idle control circuit which are prone to blowing.

Also, have you tried resetting the ECU and following the bedding in process as if you were installing a chip, which is to:
"refit the ECU in the car (you can just leave the ECU as it is and disconnect the battery for a while to reset it) and start the engine, allow it to idle for at least ten minutes and then switch off. Go for a drive, the car will run a bit “rough” for about an hour while the ECU learns the new chips settings."

[Snowey]

Thank you I’m pulling my hair out (what’s left of it) over this I’m sure it’s a simple fix when I find the culprit as it was fine until I dropped the gearbox. Exhaust and fitted the new Taiwanese petrol tank.
Yes the idle is bad fluctuating around 400 rpm Yes the Motronic unit is the 0261 200 172. I’ve disconnected the battery now and will report back. It even runs better when I disconnect the Air Flow Meter although rich. I accept there may be a small air leak in the induction somewhere as the idle was a bit erratic before. But now when you try and drive the car the engine doesn’t respond to the throttle and won’t accelerate. It doesn’t want to rev over 1500/2000 rpm.

[Speedtouch]

Have you tried cleaning out the idle control valve (ICV) and Throttle Position Switch (TPS)?

This link from E30 Zone Wiki may be of help:
https://www.e30zone.net/e30wiki/index.p ... #Air_Leaks

Also worth checking the Blue temperature sensor:
https://www.e30zone.net/e30wiki/index.p ... emp_Sensor

[Snowey]

Yes I thought I’d better go back to basics, so I reset the TPS this morning and cleaned the ICV with some brake cleaner and got it buzzing, I tried to separate the 191 connector (big mistake) I turned it but couldn’t get it apart now the temp gauge doesn’t work (will that affect things) The car is a little bit better than it was but still won’t tick over, big flat spot at around 2000rpm, spits back, kangaroo’s down the road and lacks power. Pulled the plug off the AFM and it runs fine, nice steady tick over at 750rpm good acceleration and no flat spot. Shame I can’t get rid of the AFM or at least try a proven working one. Will order a new AFM boot from BMW tomorrow and see if that makes a difference.

[Speedtouch]

If you have problems with the C191, it may be worth eliminating it and splicing the loom together, as suggested here:
https://www.e30zone.net/e30wiki/index.php?title=C191

I doubt a non-functioning temperature gauge would affect the engine, but you never know...

[Snowey]

Got the c191 apart and just a little corrosion on one of the terminals which I’ve cleaned up. Sadly think I’ve done some damage now trying the car up the road with the misfire and flat spot. On start up it makes a noise just like a loose spark plug 2 or 3 times then stops it’s at the back of the engine, checked the plugs, exhaust manifold joints but all seems ok. Could it be the head gasket blowing out the back .

[Speedtouch]

Hmm, could be, or a blowing exhaust manifold...