1991 Seville 4.9 Engine Cutting Out Intermittently at Idle

cjhudson87 · #1 (**permalink**) 10-09-08, 01:42 AM

Hey Guys,

I have a 1991 Cadillac Seville with the Multi-Port 4.9 Liter V8 and 116,000 miles on the clock.

Since day one this car has had what I would call a miss, at idle, part throttle acceleration and cruise. This miss is somewhat intermittent (i.e. it doesn't happen consistently per revolution of the motor, but it always occurs (think baaaaa-ba-baaaaaa-ba-ba-ba-baaaaaaaa-ba-baaaaa). It is most intermittent at idle and cruise, and most pervasive under light (1/8 throttle or less) acceleration. I'm not sure if this is related, but the rpms tend to hang for a while before coming down if you free rev the motor.

I have done the following work to the vehicle, with no change in the problem, despite all parts removed being in very poor (original, never replaced) condition:

Pulled Codes (None)
Plugs
Wires
Cap & Rotor
Fuel Filter
Fuel Injector Cleaning
Checked and Set TPS
Checked and Set ISC
Cleaned EGR Ports
Checked EGR Solenoid
Replaced EGR Valve
Replaced Vacuum Lines
Replaced Throttle Body Base Gasket
Replaced Fuel Injectors

Again, I have done all this work, and the problem has not improved in the slightest. There are no codes, current or in history. If anything, it seems to be getting worse. The car also seems to be getting terrible fuel mileage, I'd say about 15 freeway. Under acceleration with any more than 1/8 throttle, this thing flat out boogies! It sounds and runs great with no sign of hesitation.

Anybody have any ideas?

Thanks,
Casey

Steve_E · #2 (**permalink**) 10-09-08, 02:14 AM

I am no expert but I have a 93 eldorado with the same engine. I had a problem which sounds slightly similar to what you had and checked everything out and it was all fine, it wound up being a front motor mount, which when the engine shook a little bit, it would shake the whole car, as soon as I put the new mount in, the thing ran smoothe as can be. Maybe give that a look!?

cjhudson87 · #3 (**permalink**) 10-09-08, 02:45 PM

Thanks for the suggestion, but I just checked the mount out and it's fine. Anybody else have any ideas.

**Playdrv4me** · #4 (**permalink**) 10-09-08, 07:18 PM

That's a real brain teaser, especially with that motor since it's so reliable compared to its siblings. I thought motor mount maybe too.

NickySantoro · #5 (**permalink**) 10-10-08, 09:25 AM

OP,
In your place I would again carefully examine any possible source of a vacuum leak that you had not previously considered. The scenario described would seem to indicate a vacuum leak as the culprit.

cjhudson87 · #6 (**permalink**) 10-10-08, 09:31 PM

The motor mounts all look great...and the engine doesn't move when the miss is happening. I've tried spraying carb clean all around the motor, looking for a slightly raised idle. That's how I found the throttle body base leak and a couple of the bad hoses. I don't have a smoke tester.

The miss, quite literally, is like somebody momentarily shuts the car off and then turns it back on super fast, sometimes several times in rapid succession. Then the ISC quickly responds and raises the rpm, the cutting out stops, then the idle slowly settles and it will cut out again. It's not like it's just on one cylinder either, it seems far too harsh for that. It's like the whole motor just dies.

Are there any common electrical issues with these cars that I should be looking for? I haven't changed the coil, but I would think a bad coil would produce the problem more prominently under load or larger throttle openings.

Steve_E · #7 (**permalink**) 10-13-08, 11:09 AM

I Don't mean to keep trying to belittle this situation in any way, but another thought I just had. Are you using premium gas (93 octane)? I know when my brother borrowed my Eldorado he tried using regular and only after one tank it would idle rough and everything else, soon as I topped it off with high test it went away, but still. As far as I know these engines run best on high test fuel.

---Steve E---