Warm Start - Brief Bog then Fine

[ascott430]

Hey All.

I've been trying to track down a pesky, but not too inconvenient problem with my 91 318i. Engine cold start up starts and runs just fine. A couple cranks, smooth idle and I'm on my way.

Warm engine start up. Cranks and starts just fine but then idle hunts around after about 10 seconds. When taking off from the driveway the motor bogs but you can power through the bogging. Power through for about 5 seconds and just like a switch the engine suddenly smooths out and delivers powers normally. Engine idles fine. Any thoughts? Here are a couple things I've done...

• In line fuel check valve
• New 02 sensor - but didn't check condition of connector pins
• ECU coolant temp sensor
• Fuel pressure (pre rail) doesn't dip when engine is bogging
• ICV cleaned
• No vacuum leaks seem to be present

My next step is to look in to the fuel pressure regulator or possibly leaky injectors. Also, after replacing the sensors should I reset my ECU?

Thanks!
Alex

[bmwman91]

How is the wiring in the connector for the engine coolant temp sensor? Sometimes the wire fatigues and breaks at the terminal where it is crimped in. Usually it is very obvious, causing poor running and CELs.

I have observed similar behavior, although perhaps not as pronounced as yours, over many years. My suspicion is that it is related to the engine being warmed up already but the O2 sensor not being fully heated up again when the ECU switches over to closed loop operation.

[ascott430]

I’ll check the connection this afternoon. Also might be worth mentioning. Sometimes when the engine is bogging the CEL illuminates but then disappears after you “power through the bog”

If I can catch it with the stomp test it reads the usual 1222 code.

[ascott430]

Just wanted to bump this to see if there was any other input.

Thanks!

[Panici]

Could it be misfiring due to afterstart enrichment?

Check your coils and spark plugs.

[jsnppp]

The sudden "all is ok" switch over after 10 sec feels like something is breaking free where it was stuck. I'd check ICV again? Try also running some techron through it all and do an italian tune up? I'm lightly guessing...

[downforce22]

i believe you are experiencing the m42 'hot start' issue. You need to disconnect the grey wire/connector on your engine harness on the firewall, it should not be connected. It relates to the LLCO connection to the AFM and should not be connected on US catalyzed engines.

http://www.m42club.com/forum/index.php?topic=7757.0

My vehicle is a non-cat version and when the receptacle is disconnected the idle goes to crap. When I set the scanner up for the "cat version" and look at the engine data, reference voltage for the O2 sensor is present.

It appears to me the ECU is set up for both cat and non-cat applications and the connector is sort of a "switch" if you will. EDIT: The eprom is actually the "switch" so to speak. US Spec #1.267.356.378; Euro-spec #1.267.356.214

-If connected the DME looks for an input signal from the Idle Speed CO Pontentiometer, but it must have the Euro-spec eprom #214.

-If disconnected the DME looks for input signal from the Oxygen Sensor, but that is for US spec eprom #378.

The DME (aka ECU) part number on the case is the same for both US and Euro markets (0261200175).

I had this problem where the car would start up perfectly on cold start, but on hot starts it would barely idle/stall/not rev for about 15 seconds when it would sudden clear up. On the wiring diagrams look at page 1360-4 by the afm

[ascott430]

Awesome, thank you all for the advice. I think Downforce might be on to something here. The car had been in an accident in its life and I think the wiring harness was messed with so let me see if something was plugged in that shouldnt be.

Will report back!