I have a '93 Eldo 4.9 with 59K on it. Sitting at idle I get a stumble
which can be felt as a vibration inside the car. The rear view mirror
actually shakes. It gets progressively worse when the load is
increased by putting it into "Drive" and turning on the A/C. Neither
can be isolated as the sole cause of the stumble. Each item in turn
simply exacerbates the symptom which is just barely present in neutral
with the A/C off. The hotter the engine temp, the more noticeable is
I've been chasing this for a couple of years. I've eliminated ignition
and fuel as potential causes by the simple expedient of throwing parts
at it. EGR system is clean, valve is new. ISC motor is new. Vacuum
hoses are all new. Fuel pressure is to spec.
The fooler is that it worsens the hotter the engine gets such as when
sitting in traffic on a hot day. It's hardly noticeable when first
started in the AM. No computer codes are present ever.
Other than this the car runs very well now, but this problem is
getting steadily worse and I envision a time when it will begin to
stall when brought to a stop.
Any thoughts about what avenues to pursue now would be most welcome.
First of all I think you need to determine if there is in fact an engine
idle roughness such as a suttle misfire at idle as opposed to a drivetrain
vibration being transmitted through a bad engine or trans mount. Keep in
mind the mount doen's have to be broken to do this, the isolator can become
hardenened or in some cases softened by oil contaimantion and or heat, then
the ability to dampen normal drivetrain harmonics is lost. A deteriorating
harmonic balancer on the crank can cause low speed vibrations. Without
actually feeling this I can only throw out these off the cuff/past
experience ideas. Possibly taking a vacuum reading at idle may help confirm
or deny whether engine performance is an issue. These engine were famous
for carbon deposits forming on the back of the intake valves, this is
Thanks for the fast and detailed follow-up. It definitely is a misfire
which gets worse the more the engine warms up. I observed it this
morning, idling in the driveway, starting at dead cold up to full temp
when the fans went on. Engine shake was progressively more pronounced
that warmer it got.
I took a vac reading from two unported sources with an old gauge I've
had since Ford Y block days. It was a steady 15" until the A/C kicked
on then dropped to about 14 1/2" and the shake was more pronounced. I
had my wife sit in the car with her foot on the brake. No change so I
don't believe it is a booster leak. Had her put it into drive and
observed yet more engine shake and vac went to 14".
The gauge may be tired now and not as responsive as it could be.
The marks on the face of the gauge indicate 15" as late timing but I
don't know how relevant that is to a fuel injected engine. The timing
is on the button at 10 degrees in timing mode with RPMs under 800 as
The "carbon deposits forming on the back of the intake valves" theory
is interesting. Would symptoms of that be heat sensitive? Are there
any products or procedures that you like to remove the carbon without
taking out the oxygen sensors ?
Regards and TIA
GM makes a top engine cleaner that will clean up carbon, follow the
instructions exactly, but your vacuum is low, you need to look for a
throttle body vacuum leak or try to isolate the cylinder, if you can get
into obd short out one cylinder at a time and watch the rpm drop, the one
with the least drop in rpms, may be the culprit. If you do not know how to
use the obd thru the ac control head go to caddyinfo.com
One BIG tip about that top end engine cleaner. DO NOT USE IT IN A
GARAGE. Don't use it around your neighbors house if you want to avoid
problems either. And DO NOT HAVE THE EXHAUST POINTING AT ANYTHING YOU
WANT TO KEEP CLEAN. You won't believe how much carbon and crud will blow
out of that pipe. And the smoke generated will make you think the world
just exploded. I usually take the vehicle out into the woods or up on
the state land (unused dirt roads mainly) and run the cleaner through it
there. Still looks like a forest fire but it is out of the way. I buy
the stuff by the case and run it through the fire departments gas rigs
once a year and my own once every two-three years. Then change the oil
and filter and do a tune up.
"Shep" < firstname.lastname@example.org> wrote in message
I know of a couple folks who didn't believe me when they decided to use
some on a vehicle they had. They used it in a garage next to a nice
convertible with a white paint job and top. Took a LONG time to get all
the stink and crud off that car and out of the top. I went over and
asked him why he didn't listen when I told him about what it does. He
said " I didn't think you meant it! "
Again, many thanks. Because the vac is rock steady even though low, am
I correct in assuming it is more likely the TB gasket or intake
manifold rather than any one cylinder? I've done the power balance
test as outlined in the FSM and no cylinder stood out as the culprit.
I've sprayed around the TB with carb cleaner and also used an unlit
propane torch to try to find any vacuum leak with no success. Can a TB
have an internal vacuum leak? What are the most likely areas for a
vacuum leak? I've replaced all of the small vac hoses on the engine.
What has me puzzled is why the situation is the most pronounced when
the engine is hottest. Am I correct in assuming this is because the
fuel trim is then at its leanest and, any vac leak being constant, the
net impact of such a leak is more noticeable?
Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here.
All logos and trade names are the property of their respective owners.