Cadillac Repair: Uneven idle in 95 LT1 5.7ltr engine, mass air flow sensor, throttle position sensor

Question
QUESTION: Sir,

have a 95 LT1 5.7ltr engine with persistent uneven idle, and 'fishbiting' around 50-60mph especially. Makes entire car vibrate. Hesitation on accelleration, almost feels like a misfire, but not sure if it is. Car has plenty of power if I go WOT. At idle, lights can dim very slightly every few seconds, seemingly in time with the lumpy idle.

When in motion goes away if I make it change down to 3rd but returns once car is cruising or at steady speed again. Not sure if codes are being thrown up as have no code reader. Check engine light does NOT come on.

Mechanic has replaced following parts to try to cure it:

official Delphi optispark (problem went away for a few hours but came back, but a little better)
HT leads,
new AC delco sparks,
injectors cleaned,
coolant temp sensor,
Throttle position sensor,
mass air flow sensor,
coil, coil control module,
air filter,
timing chain and cogs,
waterpump,
throttle body intake elbow,
brand new battery,
full synthetic mobil 1 higher milage formula oil and filter change.

Still on original electrical opti harness and vent harness. Unknown when last fuel filter was changed.

Baffled as to what else it could be?

Thanks for any light you may be able to shed on this.

Keith

ANSWER: Does the air conditioning system work properly ?.

If you are low on AC refrigerant this would cause the AC compressor to constantly cycle ON & OFF excessively which you would feel like you had a miss in the engine.

You could also have a problem with the overdrive system whereas the overdrive system is engaging and disengaging excessively due to low transmission fluid and or LOW TIRE PRESSURES. Don't laugh, Check the tire pressures. I have seen transmissions rebuilt and thousands of dollars spent replacing parts and when the vehicle finally came to my facility, I drove it, Checked the tire pressures which were about 10 lbs low on each tire, Aired them up to specifications and PROBLEM SOLVED. So do not overlook the simple things.


Try this test:

1. Get the engine up to speed as to where your problem is and while maintaing your speed with your right foot, LIGHTLY apply the brakes with your left foot, Just enough to illuminate the brake lights and see if your problem goes away.

---------- FOLLOW-UP ----------

QUESTION: Thanks for the brake test tip! I will try that. :)

Ac works good. Tires at reccomended owners manual pressure.

The brake test thing, if the problem stops when I do it, and comes back when taking foot off, what does that indicate? The overdrive thing?

If it still does it during the brake test, what could that signify?

Thinking vacuum leak maybe?

Answer
The brake test thing, if the problem stops when I do it, and comes back when taking foot off, what does that indicate? The overdrive thing?

Possible intermittent switch apply/release of the overdrive switch at the brake pedal and or simply misadjusted.

Possible overdrive fault in the transmission solenoid.





If it still does it during the brake test, what could that signify.

Possible vacuum line leak in any of the vacuum lines.

Possible cracked spark plug upon installation.

Possible fuel injector fault.


Question: How does trhe vehicle idle ?.