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Low speed miss - if revved up - Engine light comes on steady

3K views 15 replies 4 participants last post by  Roger Mabry 
#1 · (Edited)
Just replaced Opti/Dynaspark with "factory" rebuilt unit. They said the
bearing failed and took out all the optics inside as car just stopped and had to be towed home. Has been on the car for 30 months working 100%.

No codes are being shown. All data in the scanner in real time is normal, except RH 02 sensor reads lower at times than LH side. No exhaust leaks. '95 LT1 from a Pontiac in a Jaguar sedan.

Service soon light is on steady after revving high or "clearing codes (even though there are no codes" in the scanner. Light then stays on steady. No EGR valve (delete) and no Cats as car is Smog Exempt in CA. Light has never been on previously when running...

Has a low to medium speed miss under load at idle it is fine. Fuel pressure is OK and no wet plugs. Inline fuel pump is almost new and filters are clean and fuel is flowing through them. Spark is good with all wires - used spark plug grounded to test.

Will test tonight with water mist to see if any wires are arcing...

Any other ideas?
 
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#2 ·
No arcing from wires shown at night

:werd:This is so odd - no codes - MIL light on steady and a miss...
 
#3 ·
Stuck in open loop with MIL light on steady

Still no codes....pulled and checked all plugs. All gapped correctly and near new NGK's - look correct coloring.

Starts fine but idles "rough" when revved and it misses around 3000 rpm and at other times in higher range. MIL light will come on and stay on when revved and both fans come on when in Open Loop.

Pulled each injector electrical plug out and their each removal made a difference that I could tell when running. Sprayed WD40 on all the vacuum hoses and other connections looking for leaks that would cause the miss and not send a code.

Using Autoxray scanner hooked up all the time and it has had codes in the past show up when other problems happened while straightening out the LT1/700R conversion problems when I bought it from the PO

Will do all the Optispark testing...it does have a Dynaspark Opti that has worked well for 30 months since the GM unit failed. Engine only has 60K and it has good even compression.

Vacuum is steady at 20 inches at idle so no mechanical problems are showing up.

Interesting that there are no comments???? No one has ever had an LT1 run rough???

 
#4 ·
Vacuum is steady at 20 inches at idle so no mechanical problems are showing up.
testing vacuum only at idle for a miss that only comes up under throttle = incorrect testing procedure. steady vacuum at idle in itself doesn't rule out mechanical problems

for all we know, a bunch of your rocker nuts are too tight, or your exhaust is plugged up or something. simple idle vacuum wouldn't necessarily reveal that stuff. datalog MAP under actual driving conditions, that'll tell you a lot more..

lots of opti problems wont throw a code. to test for 'failure' it simply counts the number of high resolution pulses, and low resolution pulses. not enough high res pulses between low res pulses = high res pulse failure. too many high res pulses between low res pulses = low res pulse failure. 96+ LT1s are a bit better at detecting optical sensor problems, as it has a crank sensor to compare to as well.

that doesn't necessarily mean the pulses are in the right spot or whatever. the cap could be arcing out, or the rotor could be hanging on by one bolt. your timing chain could have jumped. that wouldn't throw a code, and might not show up at idle

having a check engine light without any codes? that's really screwy. i have to question your scanner, but i've never seen that before.

did you check fuel pressure while revving it out?

have you tried driving it with the MAF unplugged?

how does it run when forced into open loop (unplug an o2 sensor)?
 
#5 · (Edited)
Will do the requested tests

Steveo

Thanks for the reply, will do the suggested additional testing (do not have a MAF) and post the results. Too much "fun" for today already...tomorrow is another day. Engine is a '95 Pontiac but the PCM is from a '93. I know, an "abortion" but it was working 110%
for the past couple of years.

Did get a Code 34 from unplugging the MAP sensor at idle. That shows that the scanner is working properly enough to store new codes.:LS1LT1flag:

But, a normal reading of vacuum that is steady shows "general good mechanical condition". I.E burned or sticky valves would not be steady
etc.

I did send an email to Dynaspark asking them about this problem...but they are closed due to the time differences from CA.
 
#6 ·
i have to disagree, standing idle vacuum doesnt reveal a whole ton of mechanical problems. you have to vac. test at a variety of rpm and load.

especially things like hyd. lifter valvetrain allow for weird things like valves floating out at like 2k but be fine at idle. things like exhaust restrictions might not reveal themselves until you rev it out. hell, even an engine with rediculously low compression across the board usually reads alright at idle.

the 93 ecm swap is no big thing, as long as it was done right;

keep looking and dont rule out things like your coil either, or even your coil wire.. its way easier to spark at idle with no load, or in mid-air with a spare spark plug..
 
#7 ·
A bad plug wire internal pinched conductor, ICM, or internally fractured sparkplug coil going bad etc could cause this. Tend to think electrical ground short or sensor going bad rather than mechanical. Could try swaping the O2 sensors from left to right side to see if that is causing issues.
 
#8 ·
Did all suggested testing - sent Dynaspark in for repairs

Replaced the spark plug wires, checked the coil (again) and the ICM at local store. All were good...and near new. Swapped the 02 sensors and same results.

Took off the Dynaspark/Opti and returned it for repairs....missing and
backfiring are all signs of Opti problems (again).

Opti voltage tests were good per testing procedures but nothing else is wrong with the car that shows up.

Started properly, just had an idle miss and then more missing/backfiring when revved up and then the MIL light would come on steady with no codes in the scanner and all running values are normal. It would then be in Open Loop...
 
#9 · (Edited)
Try cleaning the MAF with electrical contact cleaner like carbon tetrachloride. Clean the Idle Air Control passages not the sensor with carburetor cleaner and the throttle body. You checked for vacume leaks what about the injectors leaking or need new o Rings? Have you put a fuel pressure guage on it? Leaking or stuck or partially clogged injector? ~43 psi with vacume to regulator 36 psi without does key on psi hold for longer than 5 minutes if not the drain back to the gas tank could be bad. Seafoam or soak injectors in kerosene to clean add new o rings.


Any carbon tracking in the cap and rotor on the Opti? used to use 600 or 400 grit emory cloth to clean contacts on cap and rotor.

Throttle position sensor and Air sensor reset?

http://www.golenengineservice.com/documents/LT1 Tech Articles/Adj TPS.pdf


4th Gen LT1 F-body Tech Articles

Post #7 & #28 Carboned up Idle air control valve or passages.
*UPDATE* Tried the WD40 - IAC trick (1992 LT1) [Forums - C4 Technical] - CorvetteGuruForum.com


Have you uplugged the o2 sensors run open loop only does it miss or only in closed loop? , then the MAF.
 
#10 ·
Dynaspark/Opti is off and being tested

I do not have a MAF, '95 engine/trans and '93 PCM in a Jaguar sedan.

Injectors not leaking, were cleaned and new everything in the fuel system within past 5000 miles. I did take off the injector electrical plugs one at a time to note any difference in the running - all off made it run worse. One spark plug wire was suspect so I put on new ones on the passenger side - the hardest to put on in this car. Other side can be changed standing up outside the car with feet on the ground! Will put them on while waiting for the Opti to return. Has almost new NGK 55 plugs that were pulled, re-gapped and checked for fire at the plugs.

FP is good 46# with key on and at idle 40#. It holds pressure for five minutes then drops slowly down to about 25. Filters (three) are all new and
clean (external) and the inline pump is near new. Stock fuel regulator tests fine and has no fuel in hose.

Cannot remove Dynaspark cap due to their security fittings. They warranty the unit and even provided UPS return to their site for testing. I hope it is
their unit again, they rebuild it completely for a flat $125.00 that includes return shipping and I get a "new unit 12 month warranty again".

All the Opti R&R fears are BS, removing it from my car is easy, the stock fans pull out, I have a Meziere electric water pump that uses the stock housing = (6) water pump bolts, hoses for heater and the large hoses for the radiator and electrical connector; take off the hoses and drain the radiator coolant first. The radiator and AC condenser are left in place and there is ample room to lean over and remove the rest of the stuff. Taking off the hood is the biggest problem and installing takes two people on my car.

The serpentine belt and then the front hub comes off and with three bolts and no puller is needed. Then (3) bolts and the Opti is off in your hand after removing the plug on the wire and the coil wire. Not as easy as a SBC distributor but easy enough. Removal takes less than 30 minutes...cleaning the water pump gaskets off the block and pump takes as much time

Putting it back is just the reverse procedure - put the cam dowel in the large hole in the center, lube the O-ring and press in gently. It will lay down equally in the hole and all three bolt holes will be flush again.

If the Dynaspark/ Opti is not bad, then I have no clue what could be wrong with the car...never had the miss/backfiring and steady MIL light before. It always threw a code(s) when there were sensor or other problems.

Thanks for all the help and I will report back what else is done to make it run properly again. When it is running, it is a very fast good handling car due to the mods and 4:09 LSD gear ratio. It gets 20+ mpg at 65-70 mph on the highway even with the high gear ratio. It :LS1LT1flag:does use 91 octane fuel....
 
#11 ·
Roger, if you use paperclip jumper ALDL pin A to B does it flash a 12 (display software is OK) ?. Also is your harness custom , I know you did quite a lot of cruise control and speedometer work getting it to work on the Jag dash and switches. Like steveo I'm hung up on the SES w/o any codes, that's not normal and I have never seen it . Unless your harness is loosing +12v batt always on to the ECM and are resetting the DTC code stoage. It's been a while so I can't remember if you have TTS Datamaster DA for the '93 ECM. A datamaster log would eliminate TPS any MAP sensor failures and maybe show something. Although in Ignition HV or fuel delivery the logs are not really any help. Low speed miss is usually ignition related, fuel demand is low so it sort of rules out delivery unless pressure is bad. It looks like you already beat the ignition to death looking for any problems. Are you running the '95 injectors with the '93 ECM, was it tuned, think there's a difference of about 10% '93 was 22# injectors '95 24# , but I may be wrong. Either way it won't cause what you are seeing, would be rich if anything..
 
#12 ·
Cannot test it right now - no Opti

It has a Painless LT1/Jaguar harness. Software in Autoxray scanner is working, disconnected the MAP sensor per the GM manual, it threw DTC 34 code that erased on command.

I think the injectors are '95's, have the flow rates somewhere from the injector servicing. I have not had it tuned but some tuning was done by PO
as the VATs was removed.

Would like to get a email tune to permanently remove the EGR and AIR stuff, as well as a code from the AC due to the PCM not controlling it. Series 1 Jag AC does not work well with PCM, PCM controls the fans (stock ones). Only code that is there is the AC due to the custom stuff.

Car is for sale and I have interested buyer, next person can get the tune...
 
#13 ·
Dynaspark tested OK

They are shipping it back and my hope is they actually changed something and it will work properly again. After long testing there, they say it is 100%.

If not, I will be back for more suggestions....only have the IAC cleaning and testing to do before running out of ideas.

It was running perfectly before the Dynaspark bearing went out and tore up all the internal optics. Then it will not run correctly with a "new/rebuilt unit"???:werd:

I have confirmed twice it is being put on correctly - hard to screw that up on a '95 engine. Dowel pin in large center hole, makes sure it is flush mounted and put in the three bolts....hook up the Opti cable, spark plug wires and coil wire and start. Wires are labeled on both ends and are confirmed correct with a second opinion.
 
#14 ·
All is fine now - Dynaspark rebuilt and working

Having a Dynaspark for an Opti is good. They rebuilt it for $125.00 including return shipping.

It arrived by UPS at 4 PM and the car was running by 4:15 PM. Then the water pump, hoses, coolant and fans were installed. Still good so the hood was put back on and then the test drive. All done by 5:30 PM and on the road.

Having stock LT1 front end parts - AC condenser, radiator and the snap on fans makes working on the car very easy. Hardest part is putting the spark plugs back onto the Opti securely.

Burping the system is easy with an electric water pump, fill and watch the
valve screw for coolant coming out - wipe off, close and done.
 
#16 ·
Yes, all is very well now!

Thanks, it is great when it starts right up and runs perfectly...biggest reason to install a LSx version engine would be the upgraded ignition
and modern FI system.

Maybe on the next conversion...LS7 or LS9 ???
 
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