1996 LT1 hesitation/stumbling on takeoff at minimal throttle

I've recently had the above symptom occur on my 1996 Fleetwood.  Hot or cold engine, when minimal throttle is applied from a stop, the engine hesitates/stumbles but does not stall.  If I push the throttle more aggressively, it takes off fine.  Acceleration is spirited (3.43 rear gears).  I get 19 mpg at an average speed of 80 mph on highways.  So, except for the stumble at takeoff with light throttle, the car runs fine. Climbs to high RPM with no issues. Power seems unaffected.

I've seen this symptom described in many posts, but no one ever seems to circle back if they fix it and say what the cause(s) were. The facebook nation is replete with dodgy advice. "It's the Optispark" is frequently parroted.   I think these might be the same guys who yell "Freebird" at rock concerts.  I'm sure they mean well, but it's not very helpful. I've read "they all do that". They don't. My Fleetwood (117,000 on the clock) didn't until a few week ago. My 1996 Caprice Classic wagon doesn't do this. Neither did my previous three Caprice or Roadmaster wagons.

I've checked for vacuum leaks using starting fluid. No vacuum leaks were indicated.

David Pollack posted this video, which is from GM and outlines a systematic approach to isolate, identify, and repair the problem. See section starting at 15:30 min: [You must be registered and logged in to see this link.] It covers EGR, PCV, Throttle body, Intake manifold, MAF, Coil ground, and IAC. However, the video implies that the approach applies to 1994 and perhaps 1995. What about 1996?

I would say all the LT1 stuff is the same.

I wonder if it's as simple as just giving the throttle body a cleaning.

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I wonder if it's as simple as just giving the throttle body a cleaning.

This weekend I'll start ticking off the possible causes, test drive, and report back. TB cleaning, drive, clean the MAF, drive, clean/check IAC, drive... continue until it's fixed.

I need to figure out how to monitor imbalance in left and right bank long term fuel trims to check for an intake manifold leak using the system on my iphone (OBDLink).

dmg4 wrote:

My 1996 Caprice Classic wagon doesn't do this.  Neither did my previous three Caprice or Roadmaster wagons.

FWIW, neither my 1996 RMW (V92) nor my 1996 FWB (V4P like yours) exhibits anything like what you describe. I have put over 7k miles on each car in the past 15 (Caddy) or 5 (Buick) months. The Caddy has an aftermarket Opti (replaced right after I got the car), the Buick has (AFAIK) the original GM Opti.

I hooked up my scanner and monitored Bank-1 and Bank2 long term fuel trim at idle. The results:

Bank1 LTFT = 0.8%
Bank2 LTFT = 5.5%

Bump the RPMs up to 3000, and

Bank1 LTFT = -6.3%
Bank2 LTFT = 0.0%

Looks like I might have a leak in the intake manifold gasket.

To those out there with more experience: does this sound like enough of a difference between Bank1 and Bank to to produce the drivability issue described above?

Might be looking at an injector problem. Yes, the bank trim difference is high.

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Might be looking at an injector problem.

An injector seal leaking air, or drooling fuel, or both?  

I need to a follow-up test for vacuum on the fresh air hose.  If there an intake manifold leak, it should also show up there also according to the GM video above.

I had thought that there might be an issue with an O2 sensor, but those are relatively new with less than 10,000 miles on them.

I'll go ahead and clean the throttle body, MAF and IAC next and see if the symptoms are lessened. All were cleaned less than 15,000 miles ago, but it's easy to do. None of those would however produce a lopsided fuel trim.

""I had thought that there might be an issue with an O2 sensor,""

The fail mode with these sensors is usually slow or no response to rapid changes. They seldom shift calibration but appear to continue working.

An injector dribbling on the rich side, or one fouled on the lean side, could cause your symptoms. It is a pain, but you might try removing and "reading" the spark plugs.

Throttle body was squeaky-clean, but sprayed it down and re-cleaned it anyway. Same for the MAF. Found one dodgy boot on a vacuum line that I'll now replace, and then I'll take it out for a drive and see how it goes.

Next step might be to pull the plugs, takes notes on their condition, and replace them with new. They are a PITA.

There's one guy on the forum who knows the answer.

OK, cleaned the MAF and throttle body (neither were dirty). Replaced two dodgy fittings on vacuum lines to the throttle body and confirmed integrity of others. LTFT are now 3% for Bank 1 and 8% for Bank 2. Hesitation/stumble is unchanged.

On we go to run another test for a leak in the intake manifold gasket.

Well, at least I'm finding out what it isn't, and so far have not been tempted to fire the parts cannon it the problem. The imbalance in LTFT between Bank-1 and Bank-2 could indicate a leak in the intake manifold gasket. So, the confirming test for this was to disconnect the PCV hose, plug it, and check for vacuum in the fresh air supply hose to the right-side valve cover. No vacuum at all, both with a glove test and using a vacuum gauge.

Tested all injector seating with starting fluid for leaky seals. Nothing. So, no evidence of a vacuum leak anywhere.

On to pulling the spark plugs for a look at their condition.

silverfox103 wrote:

There's one guy on the forum who knows the answer.

Is he among the living, or do I need a Ouija Board to get a response?

Point your compass North!

or #2: Dave Pollock

The odd is that you are lean on bank 2 at idle and rich on bank 1 at 3000rpm with both swapping which on is OK but the spread staying similar.  About the only thing that I can think of is sticking at different points by the injectors bank to bank.  Long shot, but I can't think of anything else unless the Opti is bad and which cylinders see it.

Just for fun, I waited until it was completely dark and misted the sparkplug wires thoroughly from the Optispark to each plug and looked for any sign of arcing. None. On to the plugs, and then the injectors.

Pulled and replaced the plugs yesterday. Once I stopped bleeding, I took the car for a test drive, aaaaaand... it's still doing the same thing. The plugs looked fine, but had eroded to a gap of about .051 to .053 from the original .050 at installation about 15,000 miles ago. So, I've cleaned the MAF, throttle body, checked for vacuum leaks and changed the plugs. I'm at the limits of my skill, time and energy. Fortunately, not money! So, the car is off to the local repair shop and we'll see what they can do. I'll report back in a few days.

What did the plugs look like?

Did anyone appear different from its sisters?

dmg4 wrote:

Pulled and replaced the plugs yesterday. Once I stopped bleeding, I took the car for a test drive, aaaaaand... it's still doing the same thing. The plugs looked fine, but had eroded to a gap of about .051 to .053 from the original .050 at installation about 15,000 miles ago. So, I've cleaned the MAF, throttle body, checked for vacuum leaks and changed the plugs. I'm at the limits of my skill, time and energy. Fortunately, not money! So, the car is off to the local repair shop and we'll see what they can do. I'll report back in a few days.

pull the vacuum hose off the egr and plug it see it helps

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What did the plugs look like?

Did anyone appear different from its sisters?

All the plugs looked the same: Goldilocks. Not too dark, not too light, not too dirty, not too dry, Not too anything. I was hoping for a sign of a futzed-up injector, but I see nothing.

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pull the vacuum hose off the egr and plug it see it helps

DING-DING-DING-DING-DING-DING-DING-DING-DING-DING-DING-DING!!!

We have a winner!  Did as suggested above, and the hesitation/stumble vanished.  Reconnected the vacuum line and it instantly reappears.  Did it once more just to be sure I'm not dreaming, and it happened again.  Thank you Dr. Nick!

What say you on cleaning the EGR vs replacement?  It's not a big ticket item, and it has to come off for cleaning.  

For those who wanted to see the spark plugs: here they are, numbers 1-8 from left to right.  Anyone see anything amiss?

[You must be registered and logged in to see this link.]

From the plugs, it was obviously the EGR..............

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From the plugs, it was obviously the EGR

I'm still learning. What do you see on the plugs that would lead one to suspect the EGR?

I would agree that the EGR is a possible solution if the very small and convoluted passages to some cylinders are plugged. I also think the Opti could be doing it if the bearing is getting a bit loose and the gap not set at the correct amount.

There is also another thing that I know is possible because I ran across it while messing with the throttle body setup on mine after the new engine build.

On these drive by cable controlled setups they use an idle air motor to control idle speed like all the rest until they went with drive by wire stuff more recently and have the throttle motor do it instead.

Many other ones I have messed with gave a small wire gauge setting for how far the throttle plates are open at idle, but I never ran across that setting for our LT1 engines. In general, I think that gap has two purposes based on past fits and spats with the early DIY aftermarket stuff I used to have in my Mopar. One is to keep the throttle plates from sticking closed making it stick/jump every time coming off idle, and the other is to get rid of an off idle stumble, I think.

The problem shows up if the plates are too far closed and thus the idle air valve is too far open. I those conditions the airflow through the throttle plates increases fast enough to go lean before the slower idle air valve can catch up.

If the throttle plates are open further at idle the idle air control will be closed much further. The wider gap on the throttle plate means the airflow increases at a slower rate than when closed further at the start. The idle motor is not flowing as much air and doesn't have the ability to supply too much and it also needs to travel less. Basically, you just make the airflow more throttle plate dependent and less idle air valve at off idle. This also allows the throttle plate movement and rate to the PCM so it can enrich appropriately to cover both parameters.

I have read lots of places that having the idle air control at about 20 steps at warm idle speed is the best place to be on LT1 engines, and then you set the base idle speed with the throttle body so the speed is right with the idle are control at 20 steps. Once set right you can then gauge the throttle plate gap and write it down for the next time you might need to set it up.

""What do you see on the plugs that would lead one to suspect the EGR?""

It was a joke. I obviously was of no help at all in your quest to beat this down.

First time seeing this thread and was coming in to say the EGR valve is bad because I had the same problem on my old 94 but saw that Nick had already pointed you in the right direction.

I would replace the valve and clean the ports while it's off.

Old EGR valve was removed and the ports and mating surfaces cleaned up, new valve and gasket installed, VOILA, POOF, ALAKAZZAM.... she's back to running smooth as silk from get go to rev limit.  Made a 800 mile round trip to RI at no less than 80 mph on I-90 and mpg never dipped below 20.1.  That's about as good as it can get with the V4P package on a Fleetwood at that speed.  So, I am very happy.   Thank you all for your help; notably Nick who nailed it in the diagnosis.