Running rough when cold

nnunez · 07-31-2007, 11:48 AM

When I hit the highway when the truck is cold, I can feel a little hesitation. I've had a recent tune up and changed everything. I replaced the Cat as well.

I am not losing any power but its anoying to drive and have the truck chake as if there is bad grade on the road.

Has anyone any ideas why it only does it when it is cold?

75K30 · 07-31-2007, 12:13 PM

This is interesteing. Did the problem occur before, and maybe you just did not notice it, or do you believe that the symptoms are a result of something that you changedduring your tune up?
Which type of plugs did you use, and is the heat range the same? Since the vehicle is still in open loop when it is cold, it may still miss, or hesitate during the time it needs to warm up, if the heat range and / or gap was set improperly. This may go away after the truck warms up, goes into "closed loop" and symptome seem to be gone. That is simply a wild guess, but does seem to have some merit.
The other thing that comes to mind, is the simplest thing, and that is either a u-joint problem, or a something else in the drivetrain. Maybe a dry slip yoke, or even a failing center support bearing on the driveshaft. Often times these can cause similar symptoms, especially on a vehicle that has not reached operating temps yet.
Does this truck have a posi rear end? Clutch type differentials will also cause a chatter if the clutches are not conditioned, or if the fluid gets low, or dirty. Often described as 'rumble" noises and can feel like going over small high speed bumps. easy to diagnose, and worth a look. MAybe it is not related to the engine at all.

nnunez · 07-31-2007, 03:57 PM

You know, let me change the tranny fluid and the diff fluid to see if this helps.
I do think that this may be fuel related.....but I've been wrong once or twice before......

I replaced the plugs with AC Delco originals.

The problem started recently though and I thought that replacing the Cat would help but not........

nnunez · 08-01-2007, 06:57 AM

You know, I noticed that the simptom intensifies a little when I am going downhill.......
Keep in mind that I live in Houston and there are no hills here......just highway overpasses.
In any case, we have some really tall overpasses here and when i pressed the accelerator going down hill, the idling (seemed like) got a little rougher.

I assume that points to drive train?

It has a new fuel pump.

History: Last month I took it to mechanic#1 (Engine light on) and he told me that the Fuel Pressure Regulator was bad and wanted $300 to replace it. So, I took it to mechanic#2 and he ended up replacing the Cat and the light went out.....the light is still out and it passed inspection. Mechanic #2 told me that the Press Reg was fine.

I wonder if both mechanics are right.......?

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07-31-2007, 11:48 AM	#1
nnunez Senior Member Join Date: Dec 2005 Posts: 307	Running rough when cold When I hit the highway when the truck is cold, I can feel a little hesitation. I've had a recent tune up and changed everything. I replaced the Cat as well. I am not losing any power but its anoying to drive and have the truck chake as if there is bad grade on the road. Has anyone any ideas why it only does it when it is cold?

07-31-2007, 12:13 PM	#2
75K30 Senior Member Join Date: Jul 2006 Posts: 2,992	This is interesteing. Did the problem occur before, and maybe you just did not notice it, or do you believe that the symptoms are a result of something that you changedduring your tune up? Which type of plugs did you use, and is the heat range the same? Since the vehicle is still in open loop when it is cold, it may still miss, or hesitate during the time it needs to warm up, if the heat range and / or gap was set improperly. This may go away after the truck warms up, goes into "closed loop" and symptome seem to be gone. That is simply a wild guess, but does seem to have some merit. The other thing that comes to mind, is the simplest thing, and that is either a u-joint problem, or a something else in the drivetrain. Maybe a dry slip yoke, or even a failing center support bearing on the driveshaft. Often times these can cause similar symptoms, especially on a vehicle that has not reached operating temps yet. Does this truck have a posi rear end? Clutch type differentials will also cause a chatter if the clutches are not conditioned, or if the fluid gets low, or dirty. Often described as 'rumble" noises and can feel like going over small high speed bumps. easy to diagnose, and worth a look. MAybe it is not related to the engine at all.

07-31-2007, 03:57 PM	#3
nnunez Senior Member Join Date: Dec 2005 Posts: 307	You know, let me change the tranny fluid and the diff fluid to see if this helps. I do think that this may be fuel related.....but I've been wrong once or twice before...... I replaced the plugs with AC Delco originals. The problem started recently though and I thought that replacing the Cat would help but not........

08-01-2007, 06:57 AM	#4
nnunez Senior Member Join Date: Dec 2005 Posts: 307	You know, I noticed that the simptom intensifies a little when I am going downhill....... Keep in mind that I live in Houston and there are no hills here......just highway overpasses. In any case, we have some really tall overpasses here and when i pressed the accelerator going down hill, the idling (seemed like) got a little rougher. I assume that points to drive train? It has a new fuel pump. History: Last month I took it to mechanic#1 (Engine light on) and he told me that the Fuel Pressure Regulator was bad and wanted $300 to replace it. So, I took it to mechanic#2 and he ended up replacing the Cat and the light went out.....the light is still out and it passed inspection. Mechanic #2 told me that the Press Reg was fine. I wonder if both mechanics are right.......?