Loss of power with a warm engine

[laundryman]

Got a bit of a problem with my '91 940. Always starts from cold easily and goes like a train. Recently when starting her with a warm/hot engine, there is a loss of power, the engine will not rev and there is an amount of spitting & banging. Then after a minute or so the engine clears its throat and all is OK. Now these symptoms do not clear after 5 minutes and start as soon as the engine has warmed up. No faults on the engine management system, but plugs sooted up and Lambda light has come on. Had a trawl thru the FAQs but nothing there is the same as this fault.
So; when the engine is cold it will rev freely and there is the usual power and it runs as normal. When the engine gets warm it will not rev above 2500 and spits & bangs if you open the throttle and will not run at tickover, usually stalls. Temp sensor checked OK. Checked usual connections etc. Thought about fuel starvation, but engine is ok when cold. (there is no cold start injector), thought about leads/ distributor, but again runs ok when cold.

Any ideas??

[heyskull]

I'm wondering if their is a cold start valve on these engines and it is stuck.
Exactly like leaving the choke on as that would give the same symptons.

S

[CTCNetwork]

Hi,

Warm and cold idle control valves. Worth getting them cleaned and checked. There are details in the FAQ if I remember..

Des. . .

[laundryman]

On this engine there is only a bypass valve round the throttle flap valve. I believe this opens when the engine is cold. No additional valves. Will check though. Any other ideas please?
Using next doors spare car: A Grand cherokee. 20MPG!! Better find a solution soon or I will be skint!!

['88740GL]

The cold start valve or 'air circulation valve' is controlled by a bi-metallic strip heater. When its cold it should be 'open', mine in that position is open around about 3mm or so, (not checked it in this cold weather, only in the autumn months) A thermal timer supplies a voltage to the heater that surrounds the bi-metallic strip, and once heated it will close, (typically after 5 minutes of the car starting) Or when enough heatsoak from the engine is absorbed

(notice how its mounted to the valve cover?)

Normally it will never be wide open unless its a full swedish winter cold start! haha

Unbolt it from the mounting bracket disconnect the air hoses and power lead, and have a look through the unit, you might see when it is cold a tiny almost 1/4 moon shape crack of daylight through it.. This is normal...

You can squirt a good amount of WD40 inside and let it soak,, then let excess run out..probably will be full of crud, next use a thin flat rod and lever the valve inside open and close, it should have some spring to it , however i might be seized if it wont really budge...

Replace if necessary. Some older units had a screw where you could dismantle the valve,,but newer ones don't (typical) though replacements are easily available

[RealEstate]

Have you read the OBD (lambda light) codes? Easy on a '91. If the lambda has come on, there should be codes.

Suggest you check the usual culprits (Stage 0 tune-up), but first off read the codes.

All the best,

[laundryman]

The story continues.....
had a closer look at things and found the second temp sensor. Removed plug & cleaned, also checked crank position sensor plug and cleaned. Now engine ran Ok when cold & hot. Then I set off for a run of about 10 miles. half way i experience loss of power and engine would only rev to 3000. Then the fault clears after 10 seconds? When I come to a halt at junction, again loss of power and engine only revs to 3000. Blip throttle a few times and the power returns. very emabarassing at roundabouts!
I did check the cold start valve and that is OK. I guess 3000 rpm is the 'limp home' setting from the control and it is missing a sensor input?? Checking the pin 2&6 diagnostics gives no recorded fault???
Any other ideas people?
Many thanks

[RealEstate]

Have you checked the fuel relay? Always a good thing to do.

And the wiring in the engine bay inc earths? I had a loose connector last year, engine misfired intermittently. Did a wiggle test and connector on the n/s of engine bay came apart in my hand! Reseated it and no more probs.
All the best,

[laundryman]

Finally bottomed the problem!
Replaced the Engine management temp sensor (underneath no3 inlet) This improved the running when warm, but there was still the bad pickup & tickover. Disconnecting the Lambda sensor cured that. Lambda sensor replaced and the system reset and it now goes like a train!
Best resource was the FAQ section. Many thanks to those who contributed.