From bad to worse

[popo8]

LTXtech.com is my Drug...

[Injuneer]

And you replaced the rebuilt MSD Opti? I've got a very similar situation with 2 people on CZ28.com. In one case I have a data log, and it looks a lot like yours, except that is also has sporadic RPM readings in a single frame, ranging from 1,000+ to 4,000+ RPM. And the RPM on either side of that single frame is normal idle.

[popo8]

He did...he got a new DELCO from american powerhouse


Edit; ...and installed the delco.


LTXtech.com is my Drug...

[Injuneer]

I know he has the Delco now. I just wanted to confirm it replaced the rebuilt MSD. I'm looking for a pattern here.

[popo8]

I know he has the Delco now. I just wanted to confirm it replaced the rebuilt MSD. I'm looking for a pattern here.

yeah thats what inwas saying...the delco was installed and the msd gone.

guess I shouldve been more clear. sorry.

LTXtech.com is my Drug...

[Injuneer]

@TubbyFormy

A quick question - when you were having the surging problem, did you notice if the tach would show a quick "twitch" just before the RPM started to rise?

The two guys I was trying to help with a similar surging problem on another site both indicated they saw the twitch. One had an MSD Opti. The other had a Delco with about 15,000 miles on it.

In both your case and the other case with the MSD Opti, when looking at the data logs, I saw a single frame drop in RPM in many (not all) cases, just before the surge started. The low RPM reading immediately triggered a large increase in IAC counts, the RPM would peak, and in the meantime, the PCM was pulling down the IAC counts, but very slowly. The third person, with the Delco Opti did not run a data log. In the other data log though, at idle, there were single frame RPM readings of 1,000+, 2,000+ and even 4,000+ RPM, with the throttle closed and no way the engine could have done that for the 1/10th second between data frames. That is telling me the optical module was putting out erratic signals.

[SSlowBoat]

@TubbyFormy

A quick question - when you were having the surging problem, did you notice if the tach would show a quick "twitch" just before the RPM started to rise?

The two guys I was trying to help with a similar surging problem on another site both indicated they saw the twitch. One had an MSD Opti. The other had a Delco with about 15,000 miles on it.

In both your case and the other case with the MSD Opti, when looking at the data logs, I saw a single frame drop in RPM in many (not all) cases, just before the surge started. The low RPM reading immediately triggered a large increase in IAC counts, the RPM would peak, and in the meantime, the PCM was pulling down the IAC counts, but very slowly. The third person, with the Delco Opti did not run a data log. In the other data log though, at idle, there were single frame RPM readings of 1,000+, 2,000+ and even 4,000+ RPM, with the throttle closed and no way the engine could have done that for the 1/10th second between data frames. That is telling me the optical module was putting out erratic signals.

The tach would only show the true actual rpm fluctuation, where it would hang at 1500-1900, not the sporadic 1000, 2000, 5000, etc spike directly before it occurred, i replaced the msd with a new delco, car has been runnin alot better, only issue i have now is on initial start and randomly after sitting for 20 minutes and restarting, the car will crank, catch for a millisecond(act like it fired off) then stall, cycle key off and on again and it fires right up. if i prime the injectors 2-3 times with key it will not do it, so i believe there is a fuelin issue starting to show.

[Fastbird]

@Injuneer -- I've seen what you're talking about too. Several years back helping diagnose a guys car in Southern NJ, having the same random uncalled for 2000+ RPM idle surge, only his did it in gear (A4) was was really causing issues. I had TTS Datamaster logging it and about 1-2 frames before the idle bumped, the tach signal would spike to some ungodly high unumber. @TubbyFormy I bet if you were to have a quick enough acting scanner on the car it would show the same.

[SSlowBoat]

@Injuneer -- I've seen what you're talking about too. Several years back helping diagnose a guys car in Southern NJ, having the same random uncalled for 2000+ RPM idle surge, only his did it in gear (A4) was was really causing issues. I had TTS Datamaster logging it and about 1-2 frames before the idle bumped, the tach signal would spike to some ungodly high unumber. @TubbyFormy I bet if you were to have a quick enough acting scanner on the car it would show the same.

@Fastbird thats exactly what it was doing once it got warmed up fully, and i was seeing that in my datamaster logs as well, thats what the first thing i noticed was, that crazy 5987 rpm spike randomly, but when it acted up it would start doing it within seconds then boom car would rev all over the place

[Injuneer]

@Injuneer -- I've seen what you're talking about too. Several years back helping diagnose a guys car in Southern NJ, having the same random uncalled for 2000+ RPM idle surge, only his did it in gear (A4) was was really causing issues. I had TTS Datamaster logging it and about 1-2 frames before the idle bumped, the tach signal would spike to some ungodly high unumber. @TubbyFormy I bet if you were to have a quick enough acting scanner on the car it would show the same.

Oddly enough, in the data logs that TubbyFormy and the guy on CZ28.com sent me, the high spike did not cause anything to happen, except show it in the data log. Both were logging about 9 frames/sec. The thing that did show up in both, in many surges, was a slightly low RPM reading, below the programmed idle speed, that caused the IAC counts to jump from say 15-20 at idle to 40-50 in response to the one or two frames of low RPM. That is the point where the guys on CZ28.com both noticed the tach twitch just before the RPM took off.

The PCM is definitely responding to faulty cam position signal pulse rates.