98 Vortec - timing retard issue

[Truckracer]

Anyone seen this before:

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It only happens at 2400 rpm & above.

New Knock sensor.
new distributor.
spider conversion
new plugs
all four o2 sensors new
new cats
new single chamber mufflers

I'm at a loss. the truck gets 18 mpg running around 2200 rpm or less. Drops to 14.0 at 75 (2400rpm) and gets worse from there.

 

[ltxdave]

If you are not getting any knock counts, I would check and see what your MAP and TPS sensors are doing at the time of the change in timing. A drop in MAP gets you pretty close to timing levels you are seeing.

 

[someotherguy]

How many miles on it? If 100K+, have you checked for distributor wear?

Richard

 

[Truckracer]

traced it down.

It's the a/c compressor kicking in & out...

But I'm not sure if I have a problem with the a/c or if all vortec trucks do this.

Anyone else have live data scanner that would be willing to test their own truck??

 

[Truckracer]

How many miles on it? If 100K+, have you checked for distributor wear?

Richard

New distributor.

 

[Nathaniel2g]

Your a/c compressor is causing you to have a timing issue? That sounds really odd to me. The compressor working means that your system is charged properly and functioning as intended - you would have timing issues below 2400rpm as well if the a/c compressor was the issue, imo. Did you replace the whole distributor, or just the cap?

 

[michael hurd]

I wonder if the A/C compressor is on it's way out and the knock sensor is interpreting it as engine knock and killing your timing? Can you repeat your 2400 rpm driving and just unplug the compressor clutch or even the low pressure switch?

 

[Truckracer]

I wonder if the A/C compressor is on it's way out and the knock sensor is interpreting it as engine knock and killing your timing? Can you repeat your 2400 rpm driving and just unplug the compressor clutch or even the low pressure switch?

Oh yea, its repeatable. With the cooler weather now it does it as the compressor runs for the defrost.

This has nothing to do with the brand new distributor. I replace the old one in an effort to find the cause.

I'ts definelty the knock sensor picking something up when the compressor kicks in. Which goes back to my original question - do all trucks trucks do this or is something amiss with mine?

 

[Truckracer]

The compressor working means that your system is charged properly and functioning as intended - you would have timing issues below 2400rpm as well if the a/c compressor was the issue, imo.

Well, I don't think so. It does happen below 2400, just infrequently.

 

[Supercharged111]

Mine used to do it above 3000 RPM all the time. It went away with the 411 swap.