Help! Timing issues

[edsvette]

Hi New to the forum here have owned vettes for many years also new to the 81 Computer command control (CCC). 81 is in stock condition except egr valve removed. Carb is rebuilt and has new bosch o2 sensor. Vette will not pass emissions at idle but will pass at 2500 rpm. The problem I think is the timing. I set it at 6 btdc with est disconnected by the book. when I reconnect the est the timing goes to 26 btdc at idle. After a few minutes timing will go to 36 btdc idle. When I gas it the timing does not change till about 2500 rpm then jumps to 48btdc. The dwell is 30 degrees on the mcs. Idle is a little rough and hard to maintain a set idle. Drives great with no issues till you come back to idle which is not always the same rpm. Thanks Ed

 

[LannyL81]

Are the timing numbers repeatable....meaning that if after setting the timing with computer disconnected at 6°, then connecting computer, does the timing go to 36 at idle repeatly after reving the engine to 2500 rpm and seeing the timing jump to 48?

If computer disconnected, is timing at 6° again at idle?

If so, then the balancer has not shifted and I would suggest replacing the ignition module in the HEI distributor. You need the 7 pin version as long as the distirbutor has not been replaced. No vacuum advance canister right?

 

[jj_long81]

im having the exact same issue here now. i recently replaced my module. less than six month. but the other day it stalled at a light. it restarted but ran crappy. today i ordered a module and will install tomorrow. but as i was checking time i also looked at my dwell. could not achieve 30. no matter where i set it it would hang at 24.9 i hope its ignition module ill let you tomorrow evening but i suspect there may also be a tps issue for me. just wanted to know if you checked your dwell and what the results were. did the motor sound hard to start like the battery was dead after the timing went off?

 

[edsvette]

ran it today and watched the timing at start up. Started vette timing was at 6 btdc after it started to warm up timing went to 26 btdc at idle which I assume it went closed loop. Did not see the 36 btdc at idle today. Yes to your question it seems repeatable but more like 48 at 2500 rpm then 26 at idle. Also tested the temp sensor and it seems ok per manual. Correct no vacuum advance. Vette has 98,000 miles. Dwell is at 30 and tps sensor is new and set at .056 idle and the voltage increases with throttle angle. Do you have a favorite brand module. Ed

 

[LannyL81]

edsvette:
No I do not have a favorite brand of ignition module, on your own on that one. The dwell, is it varying or steady? If steady that is a problem and could be that the ecm is not functioning...which is likely to be the PROM is bad and OEM is not available new, only used, if you can find one.
Do you get a check engine lite upon start-up? Also should be lite when ecm connector at distributor is disconnected. Have you tried to see if the ecm responds to diagnostic mode by flashing out a "12"?

Suggest checking the ecm before replacing the ignition module.

jj_long81:
If you are trying to measure dwell with regards to ignition, it is computer controlled through the module. The dwell of 30 is in regards to the mixture control solenoid (mcs)...nothing to do with spark timing.

 

[edsvette]

Yes the dwell does vary. No check engine light at start and light does come on with est plug off. Went back thru emissions testing today with est disconnected to get 6 btdc timing at idle. Covered light with tape. Car passed at idle but failed at 2500rpm. I obtained an emissions wavier for now so were good with that respect. I am beginning to think that the high miles is most of my emissions problem at idle. Suspect it needs timing chain and valve job. It leaks oil bad rear main seal. Car runs great otherwise so I will drive for awhile and see how it goes from there. Thanks Ed

 

[jj_long81]

hello edsvette my car is having identical problem. so I did change my module and replaced with ac delco. no change. my car was running fine then it stalled at light. started back up but has bad idle and has gotton woarse. wont idle after it warms up and comes off fast idle. unplug my advance and idles fine at 500 rpm. plug it back in and it cranks hard runs for a second and dies. my car has 50,000 miles and in pretty good condition. my thought was a throttle position sensor but no codes are present.

 

[Tom Bryant]

Tt sounds like your problems run deeper than this but here is one thing I always do when working on the ignition ect. Your ECM will remember the adjustments it is trying to make against some other component that is not working properly. That could be somthing like a bad module or as simple as bad gas or spark plugs. Then when you correct the problem and start the engine the ECM starts to learn from that memory it was using with the bad components. I disconnect the battery for a minimum of 10 minutes to erase the memory. That way the ECM is starting over from it's base and not still trying to compensate for a bad component.

Usually in the spring the '81 is running a little rough even with fuel stabilizer in it. After getting some new gas I disconnect the battery to erase the memory. Once reconnected it is smooth again. So I'm suggesting that you always disconnect the battery when going work on the engine or ignition then you will start from a clean memory when you re- start it again.

Tom

 

[jj_long81]

hi lanny. ya I know my dwell had nothing to do with it I was just seeking a clue when tring to figure the problem but anyway the ecm is ok the ignition mod is new and so I checked tps voltage at closed position and had some 3.40 and that is what my problem appears to be. unless there is something else I should check out. what do think? now I need to know what to use to adjust the new one. thexton 362 I know but what else can be used sense I cant get one of those. thanks for all your input everyone.