jerking at cruise

[relamb]

'99 Chevy 3500 Crew Cab 4x4, 6.5TD 150,000 miles, I am the original owner since new. At 16,000 miles the turbo disentegrated and was replaced. Thereafter, when I was cruising downhill, cruise control on, highway speeds, the engine would jerk/throttle on-off/buck rapidly from idle to cruise rpm, like the cruise control could not decide if it needed to accelerate or back off. Engine would cut out and then re-accelerate. Only happened coasting downhill at that sweet spot between needing no acceleration and no backing off. Hard to do here in flatland central IN. After 3-4 trips to the dealership under warranty, taking the service manager for a ride and proving I could make it happen, they had to collect data, send it to Chevy, and then "changed out the software in the engine computer" which cured the problem. Fast forward to 2005. Had other problems and the turbo changed out at 148,000 miles because it was "worn". Shortly thereafter the same problem started as happened after the first turbo change. Only this time much worse. The bucking is extreme coasting down a slight hill with the cruise control on, and I can get it to repeat without the cruise control by holding steady on level ground.
Any ideas of what is causing this? The truck runs fine otherwise.

[rjschoolcraft]

Welcome to the forum!

I had a similar experience right before I had my pump replaced under warranty. I've also experienced the same thing while testing the SOL-D fuel solenoid driver for The Diesel Page. You wouldn't happen to be using one of these would you?

[relamb]

No Sol-D, the truck is bone stock.
One other clue - the truck is a lot harder to start now when cold (cold being less than 30F). When the block heater is plugged in, it starts instantly.

[Hubert]

Do you run any fuel additives? Changed fuel in any way? I can at times get a little shudder launching easy in 2nd gear if heavy additive concentration of lubricant like Luccas fuel treatment. I have heard its when the IP has problems metering fuel precisely at a light load. Kind of feels like it senses load then overcompensates for it and backs off then the load comes back and it cycles. I can run straight diesel fuel for a few tanks and it clears up. Could try a cleaner additive to see if it changes harshness or other affect. Have heard FSD can cause some of it as well as bad electrical connection.

[relamb]

"have heard its when the IP has problems metering fuel precisely at a light load. Kind of feels like it senses load then overcompensates for it and backs off then the load comes back and it cycles." That pretty much describes the symptoms, but it only happens at highway speeds of 60mph+ when coasting. Slight acceleration or deceleration stops it. Basically the cruise control is holding the accelerator down but there is no load. Kill the cruise control and it quits, or accelerate and it quits. But when it happens it really bucks and jerks!
But it's not the cruise control because it does it when I just hold the accelerator in mid position with my foot. By the way, have not used fuel additives and the fuel came from at least 3 different truck stops on my last long trip. I will check out all the wiring connections and look at the loose transistor possibility.

[relamb]

I've done nothing with the injectors or IP, and have 150,000 miles. Dealer has not recommended any service. Should I be replacing the injectors or adjusting the IP at this point, or doing anything else to compensate for the wear?

[Hubert]

Why its only felt at that time is because of the your automatic transmission. Torque converter locked up and its a sweet spot in RPM power band and gearing. I feel mine with manual tranny launching in 2nd gear. If I launch with a little more throttle I don't feel it. If I launch in 1st I dont' feel it either. Have read thats what the reprogram might have done for you. It overfuels just a bit to compensate for the little sweet spot precise balance. I dunno might try a higher resistance fuel calibration resistor? Might help and realatively easy and cheap.

I don't think there is any maintenance to the IP except timing (really timing chain wear which affects timing). If they are good they are good if bad replace. You can have a shop rebuild for wear too.

I don't think injectors will cause it. They do wear and performance degrades after 80-120,000 miles but they will still work.

Your truck is at the age it could use maintenance for best performance but its expensive and probably will continue to run as is for a good while longer (if the IP is not going completely south). Depends on how much money you want to pour into it.

On edit:

After driving home yesterday. I thought your jerking would be much worse condition than my shudder due to RPM's. At 60 mph + RPM 1800 + should even out a little imbalance. At ~700 rpm and the varing load of launching with no added fuel is a much harder balance of fuel rate. Probably still a fuel control problem though. Either the electronics or the actual pump can't regulate the fuel as intended.

[ 01-04-2006, 04:28 AM: Message edited by: Hubert ]

[Cowracer]

Mine does it on cruise, and as a controls engineer, it seems to me that the engineers put a little too much proportial gain in the cruise control PID algorithm.

Its a little jittery, but it is only noticable when lightly loaded due to gear lash in the drive train. I bet that new, the trucks didn't do that.

tim

[JohnC]

If you can get a Tech II on it when it's acting up, I'd be interested in seeing what was going on with solenoid closure time. I have a lot of miles in cruise control on my (x)'95 with and without a trailer, and never had that problem.

[relamb]

THE CURE - just throw money at it. Had no time to do any investigation and I wanted to get the thing reliable before a road trip. Took it in for service. Shop said there were a bunch of codes pointing to the injection pump. They put on a rebuilt injection pump, new PMD, new lift pump, and fuel filter. $1,670.00 later it runs like a charm. Hope this gets me another 150,000 miles.
Who knows what the problem originally was, because they changed everything at once.

[DA BIG ONE]

If bucking happens again suspect the pressure manifold switch inside tranmission pan.

I had this issue, and was told it was the IP. However the code (P1810) was tranny code and bucking was because tranny controller could no longer properly control transmission because pressure manifold was defective sensing wrong values and feeding them back to controller.

[Barry Nave]

They put on a rebuilt injection pump, new PMD, new lift pump, and fuel filter. $1,670.00 later

They could of at least installed a NEW IP :rolleyes: At that price :rolleyes:
Going to the trouble of replacing the lift pump,surprised they did'nt replace the OPS while they were at it. These two items go hand and hand and would of been a good ideal with the given miles.

[relamb]

>replace the OPS
By OPS, do you mean Oil Pressure Switch, or is ther some Other Potential Suprise waiting for me?

[Barry Nave]

By OPS, do you mean Oil Pressure Switch,
yes