Engine chug/sputter on sustained climb

[89blueovergrey]

I’ve recently experienced a new issue with my ’89 Reatta with 140k miles.  Upon ascending a prolonged hill at steady speed the engine chugs.  I say “chug” because it is more than a brief stutter, or roughness.  It seems the engine is completely cutting out to the point of making the car buck.  This occurs across a range of speeds – I have experienced it at 35-40mph; as well as, at 65mph while on cruise control.  Backing off the speed causes a return to normal function.  In the limited testing I have done, I have not experienced the issue on hard acceleration, even if that acceleration is on a hill.  In ‘P’ark, I have increased RPMs and had smooth running up to 5600rpm.  Above that I experienced some pulsing – vroom, vroom, vroom -- at about 0.75sec intervals.  I have run diagnostics and have no ECM codes.   I have recorded the ECM data values (at idle), but am not knowledgeable enough to know if any are out of range.

 

In the last 14 months I have replaced the ignition coil unit and ICM.  The spark plugs and spark plug cables are relatively new.  The PCV has been replaced regularly.  I have pulled the MAF sensor and it doesn’t appear fouled.  I have a can of MAF cleaner but have not yet been able to clean and test.  (Will do next.)

 

I don’t want to limit the thinking, but my thought is this may be a fuel supply issue – pump, filter?  Am I wrong to dismiss a single cylinder issue?  Anyhow, I am interested in suggestions for potential solutions or next troubleshooting steps.

Edited March 3 by 89blueovergrey
better terminology (see edit history)

[EmTee]

Connect a vacuum gauge and mount it so that you can monitor it when this condition occurs.  If vacuum drops off while attempting to hold a steady speed, I would look for a restriction in the exhaust system.  A likely cause would be a plugged catalytic converter.

[2seater]

At 5600 rpm you are probably getting into the rev limiter. If you suspect a fuel supply issue, which is possible, you really need a gauge on it to rule that out. Even better if done under load, but at least an engine off/key on and idle baseline measurements will be a good place to start. If the car is otherwise drivable, you can watch things like the MAF, ignition timing, knock sensor and other systems while underway to see if there is a change in the reading when it acts up.

[Ronnie]

4 hours ago, 89blueovergrey said:

Upon ascending a prolonged hill at steady speed the engine chugs.  I say “chug” because it is more than a brief stutter, or roughness.  It seems the engine is completely cutting out to the point of making the car buck.  This occurs across a range of speeds – I have experienced it at 35-40mph; as well as, at 65mph while on cruise control.  Backing off the speed causes a return to normal function.

What you are describing is quite often caused by a problem with the ignition system. A lot of times it's caused by bad spark plug wires. If that is indeed the problem, it will show up when the transmission is in 4th gear with the torque converter locked while going up hill at low RPMs. That is the conditions when a weak ignition system will break down.

 

To test my theory, the next time you sense the engine is starting to shudder, press the brake pedal down lightly with your left foot while holding the accelerator pedal steady with your right foot. Pressing the brake pedal lightly will cause the converter to unlock and the RPM will increase taking some of the load off the engine. If that causes the shudder to go away the ignition system is likely your problem.

 

Another test you can do in the driveway might confirm what I said above. Start the engine and put the transmission in drive. Then hold the brake tight enough that the car can't move, and start pressing down hard on the accelerator pedal to put the engine under a load. If that causes the engine to shudder it likely to be the ignition causing it.

[89blueovergrey]

Thanks for the quick replies.  I will be away from home for a few days, but will restart my troubleshooting with these ideas as soon as I return.

[garyguitar]

Hello. I started experiencing this same thing 2 weeks ago. My '90 has 147,000 miles on it. I was at dealership today. They had no clue. Ignition did not come up. Please keep me posted on status of yours. Much appreciated...Gary Brown garybrown9@gmail.com

Cleveland.

[EmTee]

14 hours ago, garyguitar said:

I was at dealership today. They had no clue.

Buick dealership?  If so, that's shameful...  Find a good local garage that has experience with the '90s - early 2000a 3800 engine.  Maybe @Matt Harwood can suggest someone.

[89blueovergrey]

Apologies for the delays on providing an update.  I have now been able to take some actions and complete some more data collection.  Since first testing, I have cleaned the MAF sensor and replaced the air filter.  I checked the resistance across the coils and had 11.2k+/-  ohms on all three.

 

On new test drives I implemented some of the simpler suggestions that were made.  I was not able to reproduce the “chugging”; however, I was monitoring ED17 – OLDPA3.  On level surfaces, and with moderate accelerations, the count did not change; however, while maintaining speed on hills the count incremented rapidly, i.e. by the hundreds.  As Ronnie suggested, I did tap the brake while in 4^th.  As described this caused a boost in RPM and the knock count stopped incrementing for a brief period.  I also monitored ED07 – O2 sensor voltage.  This ranged between 0.13 and 0.92, which from the service manual seems to be normal.

 

Next step will be to take the suggestion to replace plugs and wires.  I will provide an update when complete.  If this does not resolve the issue, I will undertake some of the other suggestions (which, for me, would require the help of a mechanic with proper equipment).  Any additional suggestions will be appreciated.

[Ronnie]

20 minutes ago, 89blueovergrey said:

As Ronnie suggested, I did tap the brake while in 4^th.  As described this caused a boost in RPM and the knock count stopped incrementing for a brief period. 

I suggested lightly holding the brake pedal down to unlock the converter which will keep it unlocked as long as your foot is on the brake pedal. Tapping the pedal only unlocked the converter momentarily and then it locked up again.

[2seater]

I would guess the large knock count bursts triggers the spark retard, ED16, which will tend to reduce power. Bad tank of fuel? Before the knock bursts, does the O2 reading tend to drop off to the lower numbers, like below .45, indicating a lean condition? ED19, short term fuel and ED20, long term fuel may also give a sense of what the ECM is trying to adjust if the fuel delivery is suspected.

[Dashmaster]

I had an issue like this years ago when I was still working on my Car. I had checked an replaces ignition modules plus and wires at least 3 times. After working on this for awhile I was about to give up. What I had finally found that solved the problem for me was the large wire harness that goes left to right  at the top of the firewall had fallen down  in the middle near the OX sensor. I pulled it up and wire tied it back in place. Problem never came back. I am guessing there was electrical induction being picked up at the OX sensor. My issue was stumbling on acceleration and pulling hills.