1998 stumble at idle (ht383e and 350 vortec)

[harle]

Hello everyone,

I have a 1998 k1500 that originally came with a 350 vortec that now has the gm replacement HT383e(has about 6,000 miles on it now and out of warranty due to time). The reason I mention both engines is because I don't remember if I had this issue with the 350 prior to getting the 383. The issue: At idle, every few seconds the engine shudders. Nothing big but enough to briefly shake the truck. Here's what's been changed and rechanged that did not fix or change the problem. This has been over the course of roughly 3 years, 3 mechanics and 1 tuner. Any help would be so appreciated as this has consumed me.

(Updated)Mpfi assembly, distributor assembly with cap and rotor, intake manifold gaskets, spark plugs, plug wires, egr valve, brake booster vacuum hose, cats deleted, heads were taken off and found some valves were not sealing correctly(not even sure how that's possible on a new engine) so that was fixed, ICM, IAC, TPS, cleaned mass air flow sensor, pcv valve, alternator, coolant temp sensor. Essentially anything that could be checked, was. I'm sure I'm leaving something out but at this point it's so far beyond my abilities. I've been on many forums looking through posts and it seems there is at least one other person that had this issue. He believed his went away after a 411 swap. One of the mechanics opened a case with GM but nothing was solved. I have inquired about the case but do not personally have the equipment to bring any new data.

Finally,
I'm not opposed to just taking it to another shop if anyone knows one in the Dallas, TX or Little Rock, AR region.

 

[RichLo]

Maybe a dumb answer but did you have any work done on the AC system? A stumble every few seconds could the AC clutch engaging and disengaging if it has been over-charged.

 

[Schurkey]

Hello everyone,

I have a 1998 k1500 that originally came with a 350 vortec that now has the gm replacement HT383e(has about 6,000 miles on it now and out of warranty due to time). The reason I mention both engines is because I don't remember if I had this issue with the 350 prior to getting the 383. The issue: At idle, every few seconds the engine shudders. Nothing big but enough to briefly shake the truck. Here's what's been changed and rechanged that did not fix or change the problem. This has been over the course of roughly 3 years, 3 mechanics and 1 tuner. Any help would be so appreciated as this has consumed me.

(Updated)Mpfi assembly, distributor assembly with cap and rotor, intake manifold gaskets, spark plugs, plug wires, egr valve, brake booster vacuum hose, cats deleted, heads were taken off and found some valves were not sealing correctly(not even sure how that's possible on a new engine) so that was fixed, ICM, IAC, TPS, cleaned mass air flow sensor, pcv valve, alternator, coolant temp sensor. Essentially anything that could be checked, was. I'm sure I'm leaving something out but at this point it's so far beyond my abilities. I've been on many forums looking through posts and it seems there is at least one other person that had this issue. He believed his went away after a 411 swap. One of the mechanics opened a case with GM but nothing was solved. I have inquired about the case but do not personally have the equipment to bring any new data.

Finally,
I'm not opposed to just taking it to another shop if anyone knows one in the Dallas, TX or Little Rock, AR region.

What is your fuel pressure?

Connect a scan tool, look at misfire counts for each cylinder, and verify the sensor data.

Maybe a dumb answer but did you have any work done on the AC system? A stumble every few seconds could the AC clutch engaging and disengaging if it has been over-charged.

I'd expect a mildly OVER-charged A/C system to have the compressor running all the time. An UNDER-charged system might cycle fairly frequently.

 

[L31MaxExpress]

Probably the idle spark variation in the tuning that helps control the idle speed. My Express van did the same thing running the stock values with the 395' cam used in the HT383. It actually always ran like that, just slightly less noticeable with the factory cam. The engine tends to have a noticeable shudder when the PCM pulls timing to help lower the idle speed. You can see it on a scanner. It will read timing advance values of say, 23, 18, 26, 20, 23, 15, 28, etc. I reduced the values about 50% around idle speed, which stabilized the timing advance and made the engine run smoother.

I am near Fort Worth, but little to no spare time at the moment.

 

[harle]

Thank you guys for the insight. I have not had any ac work done but it does it with or without the ac on. Assuming that would rule that out? I believe fuel pressure was 58 psi. The pump was replaced once several years ago. I don't have a scan tool available but there are no misfire, or any codes for that matter. The truck runs great otherwise.

I'm not sure what idle spark variation is but the guy that did the tuning attempted to even it out with HP Tuners. Now rather than a big shudder every few seconds, it's a faster less noticeable shudder but more frequently. Interesting take on how it pulls timing. That's the first I've heard that in my go arounds with this truck but it gives me hope that it's something that can be fixed.

 

[SUBURBAN5]

Did you ever resolve this?

 

[L31MaxExpress]

Did you ever resolve this?

Something you are fighting?

 

[SUBURBAN5]

Something you are fighting?

Since day one my 383 has sounded like a diesel. It idles pretty loud. closer to the oil pan it seems like. It also has the shudder mentioned above by the op. To my knowledge nothing has been a issue meaning fuel, sensors, oil pressure and intake. If it's normal, then I'd feel a little better. But everybody that see my engine says it looks like it's missing because of the shudder. I'm more nervous about the fact that I did something wrong on install...

 

[privateer]

Since day one my 383 has sounded like a diesel. It idles pretty loud. closer to the oil pan it seems like. It also has the shudder mentioned above by the op. To my knowledge nothing has been a issue meaning fuel, sensors, oil pressure and intake. If it's normal, then I'd feel a little better. But everybody that see my engine says it looks like it's missing because of the shudder. I'm more nervous about the fact that I did something wrong on install...

Did you resolve your shudder?

 

[SUBURBAN5]

Did you resolve your shudder?

Not yet I'm gonna save up some money and purchase a 411 ecm from the junkyard then take it in somewhere for somebody to install it and tune it. I've heard that calms the engine down for some reason. Haven't confirmed it yet though so I'll be the guinea pig when that time comes.