intermittent fast idle, 85 240 GLT

My 245 GLT (B21FT) has an intermittent problem - the engine races on
occasion. Revs up to about 3k in park. Provides a little too much excitement
while out driving in traffic the few times it has happened. Not sure if it's
related, but I reconnected a vacuum line to the thermal vacuum valve. The
line looked connected, but wasn't. Idled smoother afterwards, but that's
about when the engine racing started. Also replaced the exhaust system from
the turbo back. Not experienced with fuel injection or emission control
systems. Have to replace the wiring harness - figure that could be a
contributor. Recent posting about a 740 described a similar problem. Answers
suggested it could be the throttle switch. Possible this is similar problem?
Any help greatly appreciated.

Thanks in advance,
Mike

 

The coolant temp sensor supplies the signal for regulation of the idle
air control motor to the engine control unit. A faulty sensor or
connection or wiring harness problem can cause the control unit to open
the IAC all the way.

Bob

 

It's possible your wiring harness is causing this. Since you have to
replace/repair it anyway...

--
Mike F.
Thornhill (near Toronto), Ont.

Replace tt with t (twice!) and remove parentheses to email me directly.
(But I check the newsgroup more often than this email address.)

 

Thanks, Bob!

Had read about the coolant temp sensor in the constant idle system section
of Bentley. However, the troubleshooting section only discusses low idle,
not fast. Thanks for shedding light on this problem. Will be able to test
the sensor this weekend after I install the new wiring harness.

Mike

 

Having just had a wiring harness problem myself, I would definitly start
there. Harness rot will cause all sorts of weird random electrical faults.

 

What year is your 245? Are you talking about the thermal valve that's
usually blue plastic that's screwed into the head and buried under the
intake manifold?

 

How often does the high idle occur? I've had a periodic fast idle for a
few seconds ever since I bought my 240 turbo new. These days, it only
happens once every six months or so. It has never been satisfactorily
diagnosed to this day. The dealer, (Keyes Motors in Van Nuys, Ca.)
wanted to do the shotgun approach several years ago but I declined...of
course it was after the warranty period, Duhhh.

 

Thanks for all the great responses! The fast idle has happened four times.
Each time for at least a couple of minutes, always while driving. Was hoping
to replace the wiring harness today, but things didn't work out. Hopefully,
tomorrow as my alternator is acting like it wants to jump on the "list of
things to fix". The alternator gauge needle hangs just above the red on the
low end of the scale for a few minutes until the car warms up, then it jumps
to the high end of the meter. Belts look fine, although I'll replace them
just in case. Hoping this is also related to the wiring harness (crossing
fingers), but I doubt it. Started searching for advice on the alternator in
the event I can rebuild or clean something to fix it. After repairing my
wiper motor (thanks to Dave Shannon's great site), I'm hoping I can repair
my alternator, too. I'm not really holding my breath though.

 

1985. Yes, the thermal valve is exactly as you've described. Replaced a
number of vacuum lines today including both that connect to this valve. No
fast idle today, but I did notice that while the car's cold, it'll idle
smooth, then the turbo boost will slowly rise while the engine rpm's drop
and the idle becomes rough. Then, after about one second, the boost drops
back down and the idle rises while the engine returns to a smoother idle.
Just starting to look this up in my Bentley manual.

 

The alternator problem sounds like the beginning of brush failure - as it
warms up the brushes begin to make contact. The devil of brush failure is
that the warning light doesn't come on because it depends on current through
the brushes.

The brushes are part of the regulator block, which cost me $35 US the last
time I bought one. You can do a band-aid fix by removing the regulator
assembly from the alternator (be sure to disconnect the battery first!) and
bending the attachment posts for the brushes so the brushes extend a smidgen
farther. It will buy you a few weeks and confirm the problem.

Mike

 

That thermal valve is part of a system that provides cold engine
acceleration enrichment. It should pass vacuum when the engine is cold,
and not when the engine is hot. The bellows device that it's connected
to on the firewall should close its electrical contacts when vacuum
drops suddenly (as during sudden accelleration). Those contacts should
be open during normal conditions. This temporarily grounds a pin on the
Lambda computer, which enriches the mixture.

--
Mike F.
Thornhill (near Toronto), Ont.

Replace tt with t (twice!) and remove parentheses to email me directly.
(But I check the newsgroup more often than this email address.)

 

Make good sense. Last experience I had with a voltage regulator was in high
school on my '59 Ford pickup. Was pretty certain things had changed since
then, just wasn't sure how. Was going to try the band-aid fix this weekend,
but I came down with the flu. Will just order a new regulator from FCP
Groton, install it and go from there.

Thanks again.
Mike