89 Prelude Engine Dieseling

My 89 Prelude "Diesels" after turning it off. It's quite annoying.
Checking the service manual, it says that this is caused by
overheating the engine.

Checking the coolant level, there is a tan coloured sludge under the
cap.

What the hell is this sludge? and does it relate to the dieseling?

If not, what can I try to get the engine to stop.

I've adjusted the idle screw, but no luck.

Warren

 

Diesel = run on?

 

Well sometimes. It's caused by something hot enough in the combustion
chamber to ignite fuel like a spark plug. Often this is glowing bits
of carbon attached to the piston top and combustion chamber.

The old fasioned way to stop this with caruretted cars was to stomp on
the gas as you turned the key off, thus drowning the glowing particles
with raw gas from the accelerator pump.

Possibly your engine needs a decarbonizing. This can be done with
chemicals but...

You may have a head gasket leaking.

First try some Techron Concentrate
http://www.chevron.com/prodserv/fuels/techron/techback.shtml

But pulling the head off for a look-see may be a better idea.

 

on old cars, i read somewhere they trickled water into the intake while
the car was revving.

i wonder if that could be done on a FI car.

 

Yes, that was another method. Essentially steam cleaning.
I was never really sure how well it worked. To really be sure you'd
have to pull the head, see how much carbon there is, put the head back
on, do the water pour trick, then pull the head again to see how much
carbon remained. I never encountered anybody with that much ambition.

 

if it was pinging before, but not pinging or dieseling after the
"trickle down" theory, wouldnt that be proof enough?

 

What's the idle speed? Too high of an idle speed will promote "run on".

 

Exactly. I feel like I own a Tempo.

 

Guys,

Thanks for the excellent tips. One question-

I'm not much of a car guy, so I don't know where exactly the carb's
intake is. Any advice on where it is?

W

 

I've recently replaced the engine, due to the head gasket.

How harmful is the sludge? Should I worry?

W

 

I've got the Idle set as low as I can without the engine quitting on me.

I expect cleaning the carb should allow me to set the idle a little lower?

Warren

 

check the spark plugs are the correct grade. if the electrodes run too
hot, they can cause detonation without spark.

problem can be worse on old engines that have significant carbon
deposits because it raises compression closer to the natural dieselling
point. assuming the ignition & carburetion systems are ok, a good high
speed 100 mile run with some long full throttle hills does wonders to
burn a lot of this stuff out.

sludge is probably a separate issue. flush out thoroughly, flush again
with deionized water, then fill with a good quality coolant.

 

Which tells me absolutely NOTHING!

 

Jim,

I've used some carb cleaner from Wynn's in my last tank of gas. It
seems to have solved the problem.

She hasn't run this well in a long time.

I'll be posting other issues with the old girl here soon.

Thanks,

W