[Roddin Rob]

For those who may recognize me or my post, I re-built this truck a couple of years ago, including long-block overhaul. I was having troubles with the engine vibrating/idling rough once it got warm. Through several posts, I got some ideas, & each thing seemed to help, but the problem never really went away, at least not for long. So now I'm back again, armed with new information & questions (of course)! Symptom: With the engine up to temperature, in gear, stopped, the idle is so rough the engine vibrates. Get on the gas & it goes like stink. I recently built a PC interface & am running the WINALDL program. I thought I might get some more useful information. Unfortunately, I don't know what some of the acronyms mean.... here's what I've got:

FLAGS:
DRP occurred flag (flashes on/off randomly)
Rich flag (random)
BLM enable flag (random)
Clear Flood flag/not closed loop (once on, stays on)
Old Hi Gear flag (always on)
no A/C requested (always on)
AIR divert solenoid on (sometimes comes on) I don't have the AIR system on this truck.

SENSOR DATA:
40kPa MAP pressure
6.5%TPS
500-575RPM
0.480 O2
73degC coolant temp.
knock count was like 6, counting up beyond 12
rich/lean counter was like 118 counting up beyond 160
13.5v battery

These conditions were with the engine warmed-up, in gear, with the parking brake on. When the engine would surge, the RPM's would dip as low as 475 & as high as 575. I log no error codes AT ALL!

I'm starting to wonder about this problem. I'm thinking it is a component that heats up with the engine & then starts to act-up, but not fail. When the engine has been shut down for a bit, but still warm, you can start it up & have to wait for it to get warmer to start acting up again. I tested the ignition coil according to the Haynes manual, but of course it's cool by then, so it tests fine. If it were the culprit, you wouldn't know. I was thinking I might try to blow compressed air on it when it starts to surge to see if cooling it settles the problem. Then I would know it's the coil... I think I also want to try replacing the little black sensor unit at the bottom of the distributor, or the little black ignition module on the firewall. To me, one of these components is acting up. All of the EGR, IAC valves have been changed... What else is there? I've brought it to a veteran GM Mechanic twice, & no dice!

In any case, if any one knows what these terms are, like "BLM" & "DRP" could you drop me a line. I'd like to know if this data can bring me closer to a solution, or at least point me in the right direction.

Thanks,

RR.

[97Z28]

Sounds like you might have a vacuum leak. have you checked all of the vacuum lines for little pinhole leaks or cracks? you should have a hose going from the intake manifold to the MAP sensor. check that hose for leaks because if theres a crack in that line you'll get an erratic idle. theres also the TPS (throttle position sensor). if that gets out of range you'll also get an erratic idle.

Another thing you might want to do is do a compression and leakdown test just to rule out that you dont have a cracked, broken or worn piston ring.