M42 dropping revs

Post Mon Apr 14, 2014 3:39 pm

basically when you stomp on the acelerator pedal quickly it bogs the revs down and almost stalls the car. Just got the idle on the car to perfect, doesn't seem to have and breather pipe leaks or extra air getting in the system. and other things to check?

Cheers

Post Mon Apr 14, 2014 4:34 pm

got the same thing with my m40, only on a cold engine though as when the needle gets out of the blue it runs fine! replaced the rubber boot after finding it was split and i still have the same problem! *scratching head*

Post Mon Apr 14, 2014 4:46 pm

Blue sensor

Post Mon Apr 14, 2014 5:15 pm

What did you do to get the idle fine? Any chance you messed with the AFM, or "adjusted" the throttle position switch?

Post Mon Apr 14, 2014 6:12 pm

I changed out the crank position sensor as it had seen better days.

What do you mean by adjusting the throttle position sensor? I thought it was set and could not be adjusted. Does adjusting the AFM change the resistance across the pins on the TPS?

Post Mon Apr 14, 2014 6:53 pm

If you have not touched the AFM screw, then completely remove the thought from your mind. DO NOT TOUCH THE SCREW. I only mention it because some bodgers use it to mask a faulty idle.

The position of the TPS under the throttle body can be adjusted by two screws (on most engines, I don't know the M42) which allow you to set the position where it comes off Idle to Open Throttle. If it's out of position, it won't switch, and the engine won't know you've pressed the accelerator.

Post Mon Apr 14, 2014 7:02 pm

Ummm I may have adjusted the AFM screw, does that just mean i need to get a new one or is there a way to reset it?

Post Mon Apr 14, 2014 7:24 pm

As far as I know you need a Hydrocarbon meter to adjust it properly.

Post Mon Apr 14, 2014 7:46 pm

so down to the local garage and get them to adjust it?

Post Mon Apr 14, 2014 8:54 pm

That's what I would do. Unless you can borrow a meter from someone.

Post Mon Apr 14, 2014 9:03 pm

Carbon monoxide meter to adjust it, BUT only when every other aspect of the engine is in full working order.
I'd recommend having the injectors rebuilt as the next step, unless you know that they've been done in the last 50,000 miles.

Post Mon Apr 14, 2014 9:15 pm

Grrrmachine wrote:The position of the TPS under the throttle body can be adjusted by two screws (on most engines, I don't know the M42) which allow you to set the position where it comes off Idle to Open Throttle. If it's out of position, it won't switch, and the engine won't know you've pressed the accelerator.

The TPS on the M42 is on the side of the throttle body. It's a carbon track with no switches. It is number 2 on this diagram

http://www.realoem.com/bmw/showparts.do ... g=13&fg=15

and is not adjustable AFAIK.

Post Mon Apr 14, 2014 9:30 pm

just got my injectors back from injectortune on friday so there in full working order. Replaced any breather or vacume hose that looked slightly cracked, new gaskets on the inlet manifold. Anything else to check before dropping it into a garage to adjust the AFM?

Post Mon Apr 14, 2014 9:36 pm

This happened to me when I replaced my vacuum hoses, turned out to be a loose nut on the inlet manifold. Worth checking just in case

Post Mon Apr 14, 2014 10:02 pm

Can i check for air leaks by spraying carb cleaner around where possible leaks are and listen for increase in RPM? any other suggested ways to search for leaks?

Post Mon Apr 14, 2014 10:22 pm

you can also check the rocker cover gasket and the spark plug o rings sometimes a bad seal there can also causes an air leak or bad spark.

Post Mon Apr 14, 2014 11:01 pm

yes you can spray about the bay definetly worth go

another way is to smoke test the intake system. Its doable for cheap if you have a compressor and a few odds and sods

google- diy smoke tester and you will get a more expensive version but this will give you an idea of how its put together

i used a couple of old subframe nuts, celebrations tin and two hose barbs. Its basically a smouldering rag, or oil in a pressurised tin that fills up the intake- total cost nothing

Post Tue Apr 15, 2014 1:44 pm

Cheers jimmy, I've asked my mate to borrow a compressor at the weekend so will collect the bits to throw a home kit together. will post results.

I've got a feeling its just the fuelling thats wrong but will do the smoke test to double check before playing with the AFM.

Just been out to fire it up again and found a coolant leak on the upper timing case cover that bolts to the cylinder head,FFS! not Not a big job just annoying after just replacing the head gasket over the weekend.