Stumble/Stutter when hot plus intermittent rough idle

[guyslp]

Hello All,

            It's been a while since I've had time to be a regular visitor and contributor, but I'm back again because I have a problem that is driving me insane and that I am convinced is going to be a "simple fix" once the root cause is known.  The car in question is a 1989 Sedan de Ville that's in marvelous condition and has been my daily driver.

            About 2 months ago while driving along at approximately 35-40 MPH I could feel a very sudden, sharp, and brief hesitation.  The way I describe it is as a "semi-miss" as I've experienced a car with a true persistent miss before and it is far more transient than that.  At the time if you stopped everything consistently felt normal at idle.  As some time went by, idle would be intermittently rough, but you never knew when.  At the time this generally happened when the car had not been running for too terribly long.

            I have had the car looked at, but as an amateur mechanic even I know that if you can't get the car to do whatever it's been doing when you have your hands on it, and for long enough to be able to diagnose it, it's impossible to fix and that's the situation I'm in.

            As time has progressed there has been some change.   Now this behavior does not seem to happen at all when the car is cold or just beginning to warm up after a few miles of driving but only when hot.  The speed at which it occurs has changed, too, and has gone up the scale to generally not kicking in unless I'm going between 50-60 MPH, up to just under 70 MPH.

            One can force the stumbling to disappear either by letting off the accelerator or by giving the car just a bit more gas to speed it up.

             The latest change is that after the car is hot and this behavior has exhibited itself while driving the idle is more frequently becoming quite rough.  I have had the car throw an E30 code twice now when this has occurred if it gets bad enough, but it is always transient.  If the car is turned off and turned on again it consistently resolves.  It will, however, hold on until or unless the car is "power cycled."  [There was also a stored E55 (I think, I don't have my notes right now, but it was definitely an E5X) code, but I believe that was historical, and it has not been thrown again after clearing everything.]

             Over the last year or so I have replaced the distributor, spark plugs, coil, and idle speed control motor (as I had been getting E30s in cold weather - the idle speed would remain excessively high for longer than it should have).  All of these except the idle speed control motor were in place, and for many months and miles, prior to this behavior appearing and the idle speed control motor was an attempt by me to see if that would fix the issue, but it hasn't.

             Someone out there has likely experienced this exact behavior, or something very similar, and will recognize the root cause, or so I'm hoping.  Any assistance in getting this resolved would be appreciated.

Brian

[Bruce Nunnally]

I suggest you check the fuel pressure regulator, and the fuel filter. 

[guyslp]

Thanks.   Anything that's easy to check and cheap as far as taking a "throw parts at it approach" is worth doing at this juncture.

I want to be able to drive along again without this nagging fear that "this drive" will be the one where whatever it is that's causing this will finally crap out entirely.

My gut tells me that if it's one of those two it's way more likely to be the fuel pressure regulator as I'd expect a clogged or partially clogged fuel filter to be a lot less intermittent in nature.  Never hurts to put a new one on, though.

[guyslp]

By the way, if anyone has preferences as to fuel filter options or fuel pressure regulator options, please offer those and the rationale.

I don't necessarily love AC Delco and AC Delco only.  

I have used Mahle filters in the past and this one looks interesting.  Strangely, it says it has a fuel pressure regulator, but I know that's not the same thing as the actual fuel pressure regulator (which I can only find aftermarket at this juncture).

[Bruce Nunnally]

Also read through this discussion: 

1. Check for fuel in pressure regulator vacuum hose.
2. If fuel is present, replace the fuel pressure regulator assembly.
3. If a problem is found, repair as necessary.

 

[Logan]

On that era of Cadillac.....there is a engine ground that can get goofy. Cadillac has a service bulletin to dismantle and clean the ground. 

If i'm thinking clearly it is mounted to the engine block out front above the starter. 

[guyslp]

Logan,

            Thanks for this, too.   It would be a first for me on an American car, but British cars (with which I'm all too familiar) seem to be rife with ground issues.

            This is easy enough to check, and to clean up, whether it happens to be related or not.

 

[barczy01]

Check the ground stud and bracket by the starter. Problems with ECM that year if its PFI. 

[guyslp]

Hello Gentlemen.

Had a couple of insanely busy days.  This model year is throttle body injection.  I think PFI came along with the 4.9 in 1990 or 1991 (I can't remember when the 4.5 was supplanted by the 4.9).

New plug wires are on tap for Monday.  It's such a PITA to get in to install these, particularly on the side facing the firewall, that I'm having my mechanic do it.

Thanks for all the ideas, which will continue to be followed up as next steps if the new plug wires don't resolve the issue.

[guyslp]

I always like to give a "disposition report" at the end of these sagas.

Short form:  It was the spark plug wires.  Two days of driving and not a bobble in any of it.

When the set that was on it was removed two that were on the firewall side showed evidence of burning on the plug end.  I am wondering if this is just age or if there could have been a problem putting them back on when the plugs were changed last year.  I'll never know for certain.

I'm just glad things are back to normal and my daily driver is just that again.