Engine Stumbles at Cruise Until it Warms Up

I have a 98' Suburban with 232K on the engine and 5000 miles on a new DS4 injection pump. I put in new injectors the same time as the pump. Recently I noticed when the engine is cold and it is cold outside (32 degrees F to 45 degrees F) the engine feels like it will occasionally stumble or miss when driving at cruising speed 45MPH or 55 MPH. It is not severe but it is only something I can feel. I see no drop in the engine RPM when this happens. What I mean by cruising speed is that I am not under acceleration or deceleration but just trying to maintain a constant speed on level ground. This symptom goes away once the engine is warmed up (and I assume the injection pump is warmed up.) When cold, there is no stumbling under acceleration and it is not down on horsepower. There is no white smoke, there are no engine codes. I monitor the fuel pressure on the output of the filter and with the Carter pump it is about 8 psi. I have changed the filter (of course) and I don't see where fuel pressure is an issue. I have also changed to a different PMD. The new pump came with a Stanadyne module on the side of the pump and I switched to the remote PMD and saw no difference. If I was to drive to the coffee shop 15 miles away and park the truck for 10 minutes, the stumble goes away and I will not notice it the rest of the day while driving.

Any thoughts? I have fresh fuel filter, good fuel pressure, no smoke, swapped PMDs. Is there something in the injection pump that could act up when cold? It is odd that this does not get worse under acceleration.

TimK

You might try replacing the PCM temp sender. If it's flaky, it can cause the PCM to mess with the fueling when it shouldn't.

Otherwise, it sounds more like the tranny is trying to leave you. This is what I've experienced right before the 4Lxxx transmissions give up. There's an operational period between cold fluid viscosity and tolerance tightening as the temp rises. A fluid change can help, but it's only temporary. I suggest trying some Trans-Medic. If the condition improves, you will have a tranny overhaul in your near future, so plan on that. Trans-Medic, a Gunk product, can generally give you a few thousand to tens of thousands of extended miles, depending on the use, before it craps out completely.

I agree with Maverick...

At around 45 MPH at light throttle the Converter clutch applies.....When these start going away they can cause some flaky "Feelings"

Yank the dip stick on the tranny and get a look at the fluid (Color and smell it) If the fluid is dark brown and smells like burnt toast or worse......Bad ju ju.......

The 4L80 does not like junk getting into the system.....Once they start to go away they churn up a lot of crap....including metal particles........

If the oil is clean and RED it could simply be a shift solenoid that is failing...........These solenoids have a filter screen too that can plug up.....

Getting a good "OPERATING" SCAN on what the tranny is doing while it's running down the road would be good.....Then drop the pan and have a peek at whats in the pan......


YESSSS...The converter clutch can do funny bump feelings that feel like a miss....

When it's acting up, lightly press on the brake pedal, enough to activate the brake lights but not enough to apply the brakes. This disables the torque converter clutch. If it stops, that's your issue.

I recall the Suburbans are all "F" engines, so it shouldn't have EGR, but if it does that can cause stumbling, too.

Good points John
EGR that's dumping when it should not will definitely foul things.....

Same thing with the brake pedal trick.....
Good diagnostic tricks.....

Is the Burb a 1500 or a 2500 ???

2500 will be an F engine...1500 trucks will have the EGR

Get the truck on a level road

Standing start and modest throttle

You should feel the 1-2 shift....then the 2-3 shift.....the 3-4 shift and then the lockup.....The lockup SHOULD see a distinct and fairly firm feel and drop in the engine RPM (About 45 mph)

If the Lockup is lazy...spongy or "Bumpy" or non existent....Herein lies the issue.

Can be controls or a worn out lockup/converter clutch

Thank you All!!. I left out a key bit of information. I suspected the transmission myself. The Burb is a K2500 and the 4L80E was well maintained but original. I had issues with it this summer not engaging and when the stumbling started this winter, I too suspected the tranny. So, I had it rebuilt by a local rebuilder (Vancouver Transmission in Vancouver WA.). The old guy who rebuilt it showed me the worn parts he replaced. He said the clutches still had a lot of life but he had not seen a truck with warn bushings like this one in a long time. He said the tranny definitely needed a rebuild and showed me a valve with wear that could have been hanging up when it was hot. It also got a new torque converter, harness and such. The tranny is working great and I am surprised in how much less vibration I am feeling at highway speeds. I will try the brake pedal drag like you recommended to see if that makes a difference.

I think you are all on to something with the temp sensors. Sensors are not too expensive and they too are original. I have ordered the two water temp sensors and the air intake sensor.

I don't think this is related but I will throw it out here anyways. This summer I was driving from Washington State to Wyoming and while cruising along with the cruise control on (65MPH) the cruise kicked out, the engine stumbled and came back with the timing slightly different (I could hear it sounded different) and for the first time ever I had a Check Engine Light. At the next rest stop I read the code and it said "intermittent crank position sensor". That was the only code. I reset the code. The sound of the engine (timing advance) sounded normal and for the next 3000 miles of my trip, I had no problems with crank position sensor again. I did pick up a spare at the next NAPA but I have not put it in because I never got the code again and it is a bit of a PITA to get to. The CPS is not original. I replaced it when I did the time chain about 20K ago. I don't think what I am sensing with the stumble is the CPS. If it was, I would expect a code and it being worse under an engine load. However, I welcome a second opinion.

I see the forum has me as as "newbie" but I have been on this forum a long time (at least until 2005 as "TimK") but I have not been active for several years. I do appreciate the help this forum provides.

TimK

Just an FYI
Personally I would not wrangle that beast apart to put in a NAPA sensor of any kind..

I have had no luck at all with their electronics stuff.

GO WITH DELCO OEM PARTS.....

I replaced a couple sensors on my current Burb and they failed in a few days....I grabbed a Delco at the dealer.

Parts have been in there a few years now....No worries.

My son in law stuck a Napa CPS is their Jeep.....Would not work....Tried a second one...No joy....Stuck in a factory Jeep part....Golden.....

Yes, Delco electronic parts. A failing CPK can, and often does, cause issues with and without a DTC, or an intermittent one. The DTC requires a specific number of "misses" per runtime before it codes. If you're just shy of that, you may notice a miss, and not see the SES.

Your "Newbie" status will automatically promote with more posts, probably before you're done with this problem. 3 or 4 isn't it. Don't sweat it, we won't hold it against you.

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Originally Posted by DmaxMaverick

Your "Newbie" status will automatically promote with more posts,

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If you start a new user name your status reverts to Newbie. Maybe DmaxMaverick can combine your recent posts with your old user name, if that's what you want. If you're trying to hide out, well, you already spilled the beans! ;)

Is there any chance this is a weird PMD issue going away when it warms up. These things do weird stuff!

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Originally Posted by DieselDavy

Is there any chance this is a weird PMD issue going away when it warms up. These things do weird stuff!

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My first thought was PMD, but after a few moments, I remembered about the TCC or sometimes EGR as being causes of an intermittent shudder at about 45-50 mph. If his PMD was remote mounted, I would re-tighten the screws on the large power transistors on the backside of the PMD. A thermal expansion/contraction issue could be a contributor for some of these sorts of intermittent problem. But, I still think it's TCC (or EGR if equipped).

Right you are Jim,
The TCC does seem to be the most logical issue.
d