Idle timing on a cammed 97 L31

**silvergs** · 08-02-2015

Got a customer that dropped off a 1997 1/2 ton that he rebuilt the motor on and installed a "Thumper" cam. Bad choice on his part but I am trying to get it to idle and run halfway descent. My issue is at an idle it suddenly starts pulling timing and it gets down to about 0 advance. My target is around 28. What conditions will pull that much timing at an idle? He says that the oil pressure guage is wrong because when it warms up it drops to 5 psi and the check guages light comes on. Will this do it?

Thanks
Mike

**fastacton** · 08-02-2015

I don't believe the PCM has an oil pressure input, so that's probably not it. I'm not very familiar with the black box PCMs, but I'd check the ignition module to ensure it's working properly and look for knock retard as well as just seeing what the timing table is set for right now.

**Fast355** · 08-02-2015

Originally Posted by silvergs

Got a customer that dropped off a 1997 1/2 ton that he rebuilt the motor on and installed a "Thumper" cam. Bad choice on his part but I am trying to get it to idle and run halfway descent. My issue is at an idle it suddenly starts pulling timing and it gets down to about 0 advance. My target is around 28. What conditions will pull that much timing at an idle? He says that the oil pressure guage is wrong because when it warms up it drops to 5 psi and the check guages light comes on. Will this do it?

Thanks
Mike

My buddies 383 had issues with the Crank/Cam correlation and it would put his truck into a default timing mode. It would run at about 0* around idle and jump up at about 3,000 rpm to about 22*. I would start by checking the CMR with a scan tool. Get it set correctly. It has been so long since I have had a black box on my 97, but may have to perform a crank relearn as well. I know on my brothers 99 Suburban it made a noticeable difference once the relearn was performed on the fresh engine. Might also be some kind of MAP related test throwing the PCM into limp mode. I would put a mechanical gauge on the engine to test the oil pressure, no sense in trying to make an engine run well that is dying anyway then getting blamed for it.

**Fast355** · 08-02-2015

Honestly not sure how that Thumper is going to work out with the Blackbox. He should either consider an 0411 swap or drop back to a cam with less overlap like the LT4 Hotcam. LT4 Hotcam will run very well in these engines if you degree it in on a 107 or 105* ICL rather than the 109* ICL it drops in at. At 218/228 @ .050 on a 112* LSA it is fairly PCM friendly even to the Blackbox.

**Fast355** · 08-02-2015

One other thing, honestly speaking as restrictive as the stock intake on the L31 is and as limited as they are in RPM, a production LT1 or LT4 Corvette cam works about as well as anything else. I put a production LT4 cam in my Express van with the Crane Gold 1.6 full roller rockers and it would pull well all the way to 5,200-5,500 rpm where the intake gave up anyway.

**silvergs** · 08-03-2015

Thanks for the feedback guys. I know its a bad choice for a cam but it is what he put in.

I have the idle timing set at about 30 but it never gets over 10 or 11. Don't know why. I will hook the scanner up this week and see where the CMR is. I'm just trying to get this to run a little better so he can go and finish up his end of it.

Mike