1990 Cadillac Deville Vacuum pressure

ELDON DWYER

MEMBER
1990 CADILLAC DEVILLE
V8
FWD
AUTOMATIC
106,000 MILES

Vacuum pressure is not high enough(should be 22 and can't get it above 15). No leaks, no breaks in lines.
Car bucks and stalls when accelerated. Once out on highway and mpg reads above 22 can run the ac but when in town must cut ac off to avoid engine dying.
Exhaust is not blocked.

5 Replies

JAMES W.

MECHANIC
2,394 POSTS

22 inches may be spec's brand new and right out of the box but, with over 100K on the motor, your not going to get that. If you were to run a compression test you will find that is below spec's also. It doesn't mean the engine is bad, it's just showing it's age. The biggest controller of vaccum is the exhaust valves and guides. They wear and leak a little bit on the intake stroke. Not unusual. Anyway, on to your problem.
Question; Any check engine light? The stalling sounds like the idle speed control is not compensating for the A/C compressor load. The ISC unit could be carboned up and still not trigger a trouble code. Bucking and stalling on acceleration may be TPS or MAF or both. Also, when was your last tune up?

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Sunday, October 12th, 2008 AT 9:41 PM

ELDON DWYER

MEMBER
3 POSTS

Supposed to be just tuned. New spark plugs+wires, distributor. Compression is 140.
Fuel pressure is high all the time even@idle.
When do injectors get impulse to fire?
Need to know what TPS and MAF stand for. Older generation?

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Monday, October 13th, 2008 AT 10:07 AM

JAMES W.

MECHANIC
2,394 POSTS

HEY! Don't knock the older generation, I'll bet i'm older than you are.
Question: Have you recently gotten a "check engine" light while the motor was running? This is important!
The "idle speed control" (ISC) adjusts the engine rpm's to compensate for added load ie. A/C compressor, high blower fan, power steering, etc. They are prone to get carboned up and not functioning properly. They can be removed and cleaned with carb cleaner. The computer will detect if the unit is bad but, not if it's dirty.
The TPS (throttle position sensor) tells the computer how hard you stepping on the gas.
The MAF (mass airflow sensor) tells the computer how much and what temperature air is going into the motor. If any one of these is defective or "out of range) it will log a trouble code into the computer and trigger the check engine light, even if only for a second or two. This is why if have had a check engine light, even for a few seconds, is so important.
You mentioned compression is 140. Is this on "all" cylinders?
The injector pulse is triggered by the computer just after a cylinder starts the intake stroke. Controlled by the CPS (crankshaft position sensor). The length of the pulse (the amount of fuel) is controlled by the TPS. BAD OR DIRTY injectors will cause a motor to stumble on acceleration.

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Monday, October 13th, 2008 AT 10:59 AM

ELDON DWYER

MEMBER
3 POSTS

72-experience with race cars years ago and aircraft engines. This cad is driving me crazy. Comfortable car in wonderful condition. Hate to ditch it.
Engine light is on. Idle speed control is working. Comp on all cyl 140. No throttle position sensor on this model. As vacuum drops fuel pressure goes up.11"v. Fp goes to 30, 14 "v. Fp drops to 25+ or -. Computer thinks you are stepping on gas. Vacuum has to come up to reduce fuel pressure. What controls vacuum? No sensor on front damper. Is it on flywheel? Where is crankshaft detector located?

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Monday, October 13th, 2008 AT 12:32 PM

JAMES W.

MECHANIC
2,394 POSTS

72 huh, you've got me beat by a few years but, not many. Fuel pressure should increase on vacuum drop indicating acceleration. That part should be good. If I understand correctly, you said the "check engine" light was on. If that is so, can you pull a trouble code from the memory? This could be a valuable clue.