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L98 Corvette and LT1 Corvette Technical Info, Internal Engine, External Engine

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Old 11-22-2012, 08:41 AM   #1
dsandmire
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Default Low RPM Shutter/missing--Solution?

Have spent the last 6 months chasing a recurring issue with my 87 Vert [w/700R tranny]. Engine would miss if I let it coast in 3rd or 4th--hit the gas and it ran fine. Had to replace the ECM as part of an engine rebuild--continually going into limp mode and wouldnt display fault codes. Opted to swap-out the after market PROM in the old ECM with what appeared to be the original factory PROM that came in a box of "stuff" when I salvaged the car. Eventually "burnt/re-coded" the PROM with the help of a local Vette/Performance Race shop to try to cure the missing. Replaced wires, plugs, distro cap but still had the miss. Finally swapped out the "factory" PROM with the aftermarket one and the miss seems to have gone away. Not familiar with 1987 PROM technology but could there be a "bad sector" on the PROM that isnt holding or is distorting the coded parameters/ info????
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Old 11-23-2012, 11:43 PM   #2
1986coupe
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Have you modified the car?

How is the fuel pressure?
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Old 11-24-2012, 06:19 AM   #3
AirForceOne!
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Checked the fuel injectors? How about the timing?
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Old 11-24-2012, 01:11 PM   #4
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Quote:
Originally Posted by dsandmire View Post
Had to replace the ECM as part of an engine rebuild--continually going into limp mode and wouldnt display fault codes. Opted to swap-out the after market PROM in the old ECM with what appeared to be the original factory PROM that came in a box of "stuff" when I salvaged the car. Eventually "burnt/re-coded" the PROM with the help of a local Vette/Performance Race shop to try to cure the missing. Finally swapped out the "factory" PROM with the aftermarket one and the miss seems to have gone away. Not familiar with 1987 PROM technology but could there be a "bad sector" on the PROM that isnt holding or is distorting the coded parameters/ info????
Sounds like you went BACK to the original chip w/o doing anything to it. As such, your conclusion doesn't sound justified. But, I don't believe chips can be "flaky" as you describe. I wouldn't rule out some type of intermittent connection on a board/socket.

As long as the miss is gone, what's your goal at this point?
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Old 11-24-2012, 04:56 PM   #5
dsandmire
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Appreciate the inputs--my "success" with the chip swap appears to be too good to be true. Tried starting the car this am and it faultered a bit then started but immediately showed a code 13--02 sensor. Timing is about 7 degrees TDC and I have not checked fuel pressure or messed with injectors-prior to the code 13, thought was to set both fuel pressure and injector issues aside based on how well the car idles and accelerates. Swaping out the chips had provided some hope or atleast point back to the ECM [which is new/rebuilt] but now with the new code 13 going on, I am about at the end of my proverbial rope.
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Old 11-24-2012, 05:32 PM   #6
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Sounds like you should swap the O2 sensor. When you checked the timing, did you unplug the [brown] reference wire? (Of course, the ECM might be causing the problem(s). A diagnostics machine best identifies issues with the ECM itself.)

You also NEED to check the injectors. OHM the coils both cold and hot. IIRC, they should measure around 15-16....with anything "down" to 13 or 14 being acceptable. When they get into single digit resistance, they're going bad. Having evenly disbursed fuel at idle is critical for a good idle.
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Old 11-25-2012, 09:26 AM   #7
dsandmire
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Yep--did that--pulled the ref wire plug to set timing and recently replaced the 02 sensor.....the forum has been a great source of information and I also invested early on in an FSM to faciliate the engine rebuild. Up until now, I was fairly content to chase fault codes and replace the obvious ignition and other sensor parts, eg, ECM, MAS, distro cap components, plug wires, plugs. Code-wise--I have an occasional "34--MAS" even after the new MAS but it is not concurrent with the missing/shake. Most recently, I was getting a periodic 13 so replaced o2 sensor. Again, engine idles quite well warm or cold and acceleration is great. The missing/shake is consistently at low RPM in 3rd or 4th gear [700r]. I am considering taking on the injectors but my heart really isnt in it--money is not so much the concern but the thought of once again tearing down the top of the engine is not sitting well with me--especially since the car idles and accelerates. May try the resistence testing just to close the door on the injectors--or not.

I've read in the forum that replacement parts could also be a problem--in this regard, I guess both the rebuilt MAS and the ECM could be suspects. Guess I keep looking for the "silver bullet" solution that makes all this go away--is there anything else I am missing that I could check/replace???????

Plan is to swap the PROMs again and/or perhaps swap the old o2 sensor--right now the o2 fault derates the engine [limp mode] so I cant even drive the beast.

Last edited by dsandmire; 11-25-2012 at 09:32 AM.
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Old 11-25-2012, 10:44 AM   #8
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Quote:
Originally Posted by dsandmire View Post
I have an occasional "34--MAS" even after the new MAS but it is not concurrent with the missing/shake. Most recently, I was getting a periodic 13 so replaced o2 sensor.

I've read in the forum that replacement parts could also be a problem--in this regard, I guess both the rebuilt MAS and the ECM could be suspects.
Yes, there have been posts about suspect replacement parts. Bosch O2 sensors and the level of accuracy of MAF replacements. Had to diagnose those unless you have more than one part...causing different results.

With your code 34, I have to wonder if you've looked at the MAF plug? Maybe you're getting an "iffy" connection there? MAM makes a tester for this specific plug.

There's also nothing wrong with taking it in for a formal diagnosis. Electrical can be VERY tricky though. I remember one guy going nuts until he rewired the circuits ALL the way from his ECM to his MAF. But, I don't think you need to do that.

When you get normal operation at higher rpms (higher voltage), I have to wonder if you're getting some drop-out on the feed wire in the MAF plug. Clean the contacts as well as you can. Plug/unplug a few times. Then re-try.

I was having an intermittent issue with idle...then did the same. I even used dialectic grease (but was told it was an insulator...not conductor. LOL Still...somehow...the problem went away by focusing on cleaning those MAF plug contacts. And, it's been gone for 2yrs.

Also was getting code 34 at the time. I'm still running my original MAF.
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Old 11-25-2012, 10:44 AM
 
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