Hi all,

Got a 1993 940T with 311k on the clock and the car seems to be running randomly rough at idle until the car warms up.

I just replace the Air Idle control valve, Fuel pressure valve fairly new, new fuel pumps (both)

Like I say it has happened randomly but I know it is not normal.

Any ideas on what this could be ?

Thanks,

Dear petiboy,

Hope you're well. I'd suspect one of the following:
(a) dirty/corroded coil contacts (corrosion may be a few atoms thick, too little to be seen with even a powerful magnifier)
(b) failing ignition power stage or fuel injection relay (aka radio interference suppression relay)
(c) failing fuel pump relay (white, rectangular, on relay block behind the front shifter console ashtray; see FAQs for access).

Some electrical components fail abruptly and totally. Others "go intermittent": they work, but not reliably.

Faulty operation that "smoothes-out" as the engine warms, suggests a weak electrical connection, that improves when higher temps cause metal to expand, narrowing a gap.

If you have spare parts, known to be good, replace the above-noted items one-at-a-time and see which one (if any) solves the problem.

There could be turbo-related malfunctions. I don't operate any turbo-equipped cars, so will defer to the Board's turbo experts.

Hope this helps.

Yours faithfully,

Spook

Hi Spook, I am following and completing the remedies you have suggested, but I wish to wire a secondary power source for the fuel pumps, as they on more occasions don't switch on when the key is first turned on although they will eventually come to life, I have fund a direct power source will have the pumps running. What I need to know is where I can pick up a power source which will come on and off with the key and I'll fit an inline on/off switch just as a back up ( belts and braces) just to keep the wife happy.

Check the socket on the relay board for the FPRly. That relay can run HOT. I had a situation once where the relay overheated, melted the board and the connections never fully engaged because they were not held in place in the board sockets when the relay was R&R'd. They only made contact when they felt like it.

How old is the relay? Over time, the contacts can become pitted which will result in intermittent contact.

Lastly, since it is a cold engine issue. Have you checked if you have a cold start injector and is it working?
--
1999 E320 1994 F150 (awaiting a new engine) 1989 560 SL 1994 945T (daughter's) 1986 242Ti (awaiting a B230FT transplant) 1968 GT500KR (under slow restoration) 2007 HD Nightster (son's ride)

"as they on more occasions don't switch on when the key is first turned on although they will eventually come to life"

You do realize that the .5-4 second burst of fuel pump activity at KPII does not exist with all model and all ECU's, right. It is not a definitive test of anything working...or not.

Have you jumpered the relay to see if the pumps always run?

Thanks spook,
My wife had a cat named spook until I could breathe anymore.....

I replaced the radio suppression relay a few months ago also, fuel pump relay has been replaced 1/2 dozen time and I have a spare to check this out.

If its the coil, is any contacts to clean ?

Thanks

Dear Petiboy,

Hope you're well. As Turbo-equipped models use Bosch ignition and fuel management, the coil is a cylinder - aluminum or black/sky blue plastic - mounted on the passenger-side strut tower brace (N. American models) and next to the air conditioning accumulator/drier (large aluminum cylinder). On a non-turbo model, the coil has three wires, two thin ones on either side of the coil wire, that goes to the distributor. I do not operate a turbo-equipped car, so do not know if the coil is differently wired.

To clean the contacts, apply an aerosol corrosion-remover or use a "scrubbie" pot cleaner, fine steel wool, or very fine sandpaper (300 grit or higher).

Any of these will remove corrosion and leave bright metal exposed. Be sure to clean both the contacts on the coil, and the contact on the wires, that attach to the coil.

After cleaning, coat the contacts with a thin layer of di-electric ("tune-up") grease. I would not use any other grease, e.g., bearing grease, while lithium grease, etc.: these are not specifically made to be used on electrical contacts.

It might also be worthwhile to remove and inspect the distributor cap and rotor. If those show signs of corrosion and/or pitting, they should be replaced.

Another - far less likely cause - is the ignition control module. This unit is mounted above the driver's footwell, and so not easy to access. Few failures have been reported.

Hope this helps.

Yours faithfully,

Spook

Thanks Spook I will give this a try