740GL symptoms: Rough Idle & Stall while cold

[JayR]

1990 740GL, 180K miles.

Starts up great, but while the engine is cold the idle is rough, and
without a tap or light pressure on the gas pedal, the car sputters down
and stalls. After she gets fully hot (seems to be about 10 minutes),
the problem disappears and the idle is fine.

I was thinking maybe dirty throttle bodies, but it seems if that was the
cause the problem would not disappear when the car is hot.

Could these symptom point to a dead or dying AMM? Where is that part
located? Reading here suggests the culprit could also be the Radio
Suppression relay? I replaced the O2 sensor not too long ago, so I'm
ruling that out.

Any ideas? Thanks in advance,
Jay R.

 

[Michael Pardee]

1990 740GL, 180K miles.

Starts up great, but while the engine is cold the idle is rough, and
without a tap or light pressure on the gas pedal, the car sputters down
and stalls. After she gets fully hot (seems to be about 10 minutes), the
problem disappears and the idle is fine.

I was thinking maybe dirty throttle bodies, but it seems if that was the
cause the problem would not disappear when the car is hot.

Could these symptom point to a dead or dying AMM? Where is that part
located? Reading here suggests the culprit could also be the Radio
Suppression relay? I replaced the O2 sensor not too long ago, so I'm
ruling that out.

Any ideas? Thanks in advance,
Jay R.

You had it right the first time - classic for throttle body glop, including
the lack of problem when warm... I don't know why. Especially clean the
passage at the bottom that goes from each side of the throttle butterfly to
the knurled idle adjustment screw. I count the turns as I screw the idle
adjuster in, then remove the screw for cleaning and reinstall it and set it
the same number of turns from closed.

The Idle Air Control valve under the throttle body also needs cleaning at
the same time. Expect to use half a can or more carb cleaner for the pair of
them, and you should be good for a couple of years.

 

[User]

1990 740GL, 180K miles.

Starts up great, but while the engine is cold the idle is rough, and
without a tap or light pressure on the gas pedal, the car sputters down
and stalls. After she gets fully hot (seems to be about 10 minutes),
the problem disappears and the idle is fine.

I was thinking maybe dirty throttle bodies, but it seems if that was the
cause the problem would not disappear when the car is hot.

Could these symptom point to a dead or dying AMM? Where is that part
located? Reading here suggests the culprit could also be the Radio
Suppression relay? I replaced the O2 sensor not too long ago, so I'm
ruling that out.

Any ideas? Thanks in advance,
Jay R.

Old plugs, bad wires, restricted injectors, vacuum leak, what have you
checked? What codes sre present that aren't triggering the light?

Bob

 

[mar]

Old plugs, bad wires, restricted injectors, vacuum leak, what have you
checked? What codes sre present that aren't triggering the light?

Bob

Sorry, posted before checking. I haven't had a free weekend in a while
to even pop the hood, so the car keeps getting gradually worse. I
wanted to get a list of all the possible things it could be based on the
symptoms, gather the parts this week, and then tackle repairs in order
of likelihood this weekend.

Hopefully it'll be the throttle bodies like Michael Pardee suspects --
but I appreciate your other suggestion and will check into those areas, too.

 

[mar]

You had it right the first time - classic for throttle body glop, including
the lack of problem when warm... I don't know why. Especially clean the
passage at the bottom that goes from each side of the throttle butterfly to
the knurled idle adjustment screw. I count the turns as I screw the idle
adjuster in, then remove the screw for cleaning and reinstall it and set it
the same number of turns from closed.

The Idle Air Control valve under the throttle body also needs cleaning at
the same time. Expect to use half a can or more carb cleaner for the pair of
them, and you should be good for a couple of years.

Michael,

Thanks for these tips. I cleaned the TB today and it fixed my idle
problem. I was amazed at how stubborn a lot of those deposits were.
Took several applications of cleaner and lots of scrubbing with the
toothbrush to get them off. This was the first time I'd ever cleaned
the TB, and I've owned the car from 60K to 180K. I think it's a good
bet it's the first time it's ever been done.

Also replaced the crank position sensor while I had the hood up.
Hopefully this will cure my hot-start problem.

 

[Jon Robertson]

maybe glug causing the throttle body to stick ,when warm it would release
easier .Clean the electronic idle control too it gums up easy .