Carrera stumbling and backfire

[Oregon Cab]

After 3 years of near flawless daily service, after an hour drive home up the final hill, my 86 3,2 starts stumbling, jerking and backfiring in second. Clutch in, it idles fine and starts fine. Later in the garage, the idle begins smooth then slowly starts to stumble as well. When I goose it, she coughs and sputters hard and if pressed over 3000 will backfire loudly running rough under a load. This is quite loud and raucaus from the Monty. I have a 1 yr old fuel pump, just tried a new dme relay and the cap and rotor look quite serviceable.

A search of the forum here indicates either plug wires or flywheel sensors. OBTW my tach has also been bouncing 500-1000 over after warming up for the last 5 months. A new alt-VR improved my normal voltage readings but did not fix my tach issues. I will pull my wires and plugs this week.

Seem strange that this came on all of a sudden from purring to coughing. Almost like bad gas or something. Ideas?

[gregwils]

Ok, this isn't my area of expertise, but after reading enough posts I can probably suggest a few things. My first guess is a vacuum leak. A hose that cracked or split could cause a quick change in performance. If you can spray some carburetor or brake cleaner around the intake manifold and around the back of the engine, and if the idle picks up speed, it's a vacuum leak.

You may also want to check the cylinder head temperature sensor. You can test to see if it's good, check Bentley. Also, make sure it's the newer two wire type. I don't think the car would run if the speed or reference sensors are bad, however, if they are old (protective cover is brittle, temp sensor too) then I would add them to your to-do list. You can get BMW parts for speed/reference from our host at half the price of Porsche part numbers. Do a search.

[kidrock]

when was the last time the fuel filter was replaced? I'd check the easy stuff first.

[DRACO A5OG]

+1, Filter, another pelican went thru the exact same thing. Found it was a clogged filter.

[stlrj]

When was the last time the ignition cables were replaced? More easy/basic stuff you do before you get over your head and overwhelmed.

[Oregon Cab]

Easy cheap stuff. Works for me. It all makes sense what has been suggested. Will report back tomorrow after trying these things.

[rusnak]

I would be curious as to fuel pressure, as already noted.

[Oregon Cab]

Replaced DME relay but no improvement so put old one back. New fuel filter had to blow out fuel but not terrible.... new one is clearer. Found 3 disconnected hoses, Filter box, small braided hose behind Air Flow box and large black hose (lower one from the oil filler area) to top of manifold was broken at the clamp. Amazing. Still stumbles. Changed fuel filter, cap and rotor. Starts and idles very smooth and strong but still stumbles and farts at 2900 and will not increase. Checked readings at the cht, speed and reference sensor seems to be ok. Tested all plug wires (2.9-3.0) and coil wire was corroded at distributor plug and could not read! Got a new connector and cleaned screw-ins. Wire checked out without caps. Will replace spark plugs, reset dist cap and wires and test Air flow meter for ohms while pushing flapper. My tech suggests I don't dismantle it.

Assumptions: 1. Starts and idles great to 2500 then hits the wall stumbles and farts. Vacuum leaks would show at idle. 2. Backfire at the tail and stumbling under a load indicates a firing and timing issue or fuel. Caused me to double check wires and replace cap and rotor. Plugs next.

Seems like when I had my 74 tii, this kind of issue was always in the timing or burnt parts. I am surprised these symptoms are such a mystery.

[kidrock]

you read my mind, I'd be checking the plugs next.

[Oregon Cab]

No go. Still stumbles spits and pops at 26-2900. New plugs, cap, rotor, fuel filter, coil checked strong, new coil wire ends, all wires checked. Idles strong, starts easy, but backfires out the tail.

Put a dwell on the neg of the coil and grounded to the body. Reads zilch at all speeds. I am told this means that the coil is not gettting the input instructions from the dme.... is this right? If the dwell reads properly then the problem would be from the coil wire to the plugs.
About ready to take it in.

Got lots of experience in tune ups this week though.

[Tippy]

As I have the same problem, check the AFM wiper track for wear.

Here's how to do it:

Air Flow Meter (AFM) - from "The 944 Motronic DME" by FR Wilk* ©2001 www.the944.com

[rusnak]

The 3.2 Carrera has 12.v on both sides. Negative test lead goes to the engine mount bolt. Re-check fuel pressure at 2600 rpm.

[Oregon Cab]

Yup, used the bolt with the brown wires under the cht sensor tree. Will try again. If flat does this mean funky dme signal? I will look into checking pressure although I do not have a gauge.

What are the symptoms of a worn track on the AFM? Did this fix your stumble and farting? Symptoms of a CHT problem?

Remember: idles strong, full tune up from coil forward.

[Tippy]

Car idles ok but right off idle up to mostly 2000 RPM nearly every gear, it stumbles and bucks.

After I did the test by the link I posted, sure enough, the first 10 degrees of travel produced 0 voltage increase.

I should add too, push the throttle to about an eighth or quarter normally driving and while under boost, the car runs great.

[stlrj]

Originally Posted by Oregon Cab

Tested all plug wires (2.9-3.0) and coil wire was corroded at distributor plug and could not read! Got a new connector and cleaned screw-ins. Wire checked out without caps.

Testing ignition wires with a voltmeter only tells if you have continuity and resistance and nothing about the dielectric strength of the insulation (with hidden cracks) and rubber boots that often offer paths of spark leakage.

The only true test of ignition cables would require the use of megger test equipment which is very expensive and beyond the capabilities of most home mechanics. If you google "megger testing" you will see what I'm talking about.

About the closet test you could perform at home on your ignition wires to simulate a megger test would involve the use of water sprayed on your ignition wires while the engine is running in order to provoke a miss at a crack or weak area of the insulation.

Unless such a test has been performed, there is no way of knowing if your cables are any good and the voltmeter test acts as no more than a placebo.

Simulated megger test: plugs gapped at 0.060"




Cheers,

Joe AKA "stlrj"

[kodioneill]

Cylinder head temp sensor check values with a VOM.

[stlrj]

Originally Posted by Oregon Cab

Symptoms of a CHT problem?

The usual failure mode is an open circuit which would signal the DME to go full rich resulting in hard starting, black tailpipe smoke and sluggish performance if you could get it to run in that condition.

[Oregon Cab]

Yesterday at the suggestion of my tech, I poked the door to the AFM to see it swivel a couple of times. I pushed in what seemed to be a poorly seated #6 plug cap. I decided to verify that my wire to cap order was correct. I removed and reseated my cap. All looked in order and so I started it right up. To my suprise it roared clean to 4500 with a small sputter and fart instead of the customary 2900 with lots of complaining. I hopped in and took it up the street like normal with a small fart and putt only at 4300. Much improved. I quickly came back and parked it for the night.

Then this evening I pushed open the AFM flapper and with my finger wiped some black oily residue from just inside reaching in with my index finger. Sort of gassy oily not heavy or rocky. Dirty. I started it with the airbox off and it could barely start and idle with backfires, stumbles and putts.... worse than before I did all this work over the last 2 weeks. Could not even rev it to try to stabilze idle. I will retry a new relay but now I am really confused. Where di that gunk come from? I will pop the AFM off and carefully wipe it out with sparing bursts of contact cleaner.

WHATTTT?

[kodioneill]

One more time CYLINDER HEAD TEMPERATURE SENSOR!!!!!!!!!!?

[Tippy]

I agree the CHT could be your problem, but have you did the Airflow Meter test?

Any voltage drops throughout the barn door sweep will tell the DME to cut fuel momentarily causing sputtering.