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

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Old 12-16-2010, 10:53 AM   #1
fc_soldier
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Default 1985 Rough Idle tried everything...Ideas?

Alright i have a 1985 corvette at my house, its been a basket of problems. I'm down to a surging idle and tested everything I can think of. Its only when idle drops and car is warmed up. I do get a code 42 EST malfunction. I think its ghost fault or caused by something else.

I cleaned the MAF, tapped it, and unplugged it. It doesn't appear to be it.

Throttle Body is clean and cleaned the iac housing. I cleaned the sensor. Haven't replaced yet the sensor.I checked gaskets and replaced nothing there.

I gave it new plugs, wires, cap and rotor. All were in bad shape needed to be done. The gap was wrong on the plugs.

I replaced driver side O2 after a code popped for it. All good there.

I checked timing. Its good. I heard of ring slipping but how can you tell?

I tapped on the ecm to see if I get stumble and nothing there.

I checked fuel injectors. The ohms check passed. It could be there spray pattern messed up? (but wouldnt it act up even in cold start just before my problem?)

I replaced battery and alternator. The alternator was a 130 amp from autozone, i returned it after it didnt fix. I put my battery in out of my vette ran good for 40 mins than my problem came back. So battery maybe not issue ( I almost feel like it maybe both alt and battery both screwed together) I had it tested at autozone and it passed everything.

I replaced the EST module no change.

My observations....
I can take any electrical load off the car such as EST, Voltage regulator, or whatever drawls current and it will run good. I plugged it in and it runs crappy. It seems to me something maybe drawling to much current at hot idle (650rpm) I thought maybe the battery has a bad cell still within specs but once it loses its surface charge it acts up, along witha weak alternator.

Help me fix this so I can stop fixing it in the snow. Nothing like a snow covered throttle body change.
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Old 12-16-2010, 01:08 PM   #2
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If your theory makes sense I would think voltage must change between running good and bad..
You should have around 13.7 volts with the engine running and charging system working, first easy check.

If so there are only 4 things I can think of.
Battery
Altenator
Regulator
Grounds al of them starting at the battery
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Old 12-16-2010, 02:56 PM   #3
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Check your EGR. It may be stuck open.
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Old 12-16-2010, 03:30 PM   #4
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Quote:
Originally Posted by ddahlgren View Post
If your theory makes sense I would think voltage must change between running good and bad..
You should have around 13.7 volts with the engine running and charging system working, first easy check.

If so there are only 4 things I can think of.
Battery
Altenator
Regulator
Grounds al of them starting at the battery
Readings

12.15v Sitting Battery
12.25v Sitting Alt
14.90v Cold Start Idle 1k Battery
15.10v Cold Start Idle 1k Alt.
14.35v-14.9v Idle 430-720 155 degrees (during bad idle my issue)
14.75v Idle 430-720 155 degrees (during bad idle my issue)


I replaced the voltage regulator...His last one was bad (flickering lights)
I checked att the main grounds and they are good.

Im tring to avoid buying a new bat and alternator on a hunch. Battery was replaced at some point within the last year.

Last edited by fc_soldier; 12-16-2010 at 03:32 PM.
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Old 12-16-2010, 03:31 PM   #5
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Quote:
Originally Posted by Shark Racer View Post
Check your EGR. It may be stuck open.
Is there a way to test it? Plugging it or something? His looks new but who knows.
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Old 12-16-2010, 04:52 PM   #6
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Check to see whether or not it "holds" a vacuum. Holding in the case of my 87 means that it will pump up and quickly drop. A blown EGR valve won't even pump up.

Operate the valve manually and see if the idle gets any worse.

My 78 had a bad idle due to a stuck EGR valve, and when I pulled the valve off I could tell - tons of carbon buildup. Cleaned it up and the car idled great.

Not so easy on an 87 though, cuz the plenum has to come off and that's a pain.
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Old 12-16-2010, 08:05 PM   #7
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Does the problem go away if you disconnect the timing connector?
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Old 12-16-2010, 08:53 PM   #8
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i had the same problem change everthing to on my 1991 i had a ajustable tps put in an the car is fine now hope this helps
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Old 12-16-2010, 08:58 PM   #9
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Quote:
Originally Posted by Paul Ruggeri View Post
Does the problem go away if you disconnect the timing connector?
Un plug and re install the connector Paul mentioned here. That is code 42. That should clear up the 42, unless the wires are shorting out behind the block.
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Old 12-16-2010, 11:07 PM   #10
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Quote:
Originally Posted by Paul Ruggeri View Post
Does the problem go away if you disconnect the timing connector?
yes but goes away if I disconnect anything like the est or the voltage regulator. It seems if I take ANy load it will clear it.

Ive even replaced the EST and still was there. Maybe the wiring like you said?
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Old 12-17-2010, 10:59 AM   #11
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I think there is a ground wire(s) on the back of one of the heads on an 85. I'd check those. Also check the wiring to the module in the distributer.
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Old 12-17-2010, 12:24 PM   #12
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Quote:
Originally Posted by Paul Ruggeri View Post
I think there is a ground wire(s) on the back of one of the heads on an 85. I'd check those. Also check the wiring to the module in the distributer.
Will do Ill check those.
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Old 12-17-2010, 12:25 PM   #13
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Another thing his oil pressure gauge is not reading right 80 psi the whole time. I think the sender is bad if it is will it cause problems?
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Old 12-17-2010, 06:55 PM   #14
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Just a couple of thoughts and observations, for what they are worth..

The voltages you listed are goofy. A fully charged "good" battery should measure 13.2 volts (at least a lead acid battery..). In a car, hooked up to Some load (dome lights, etc) it'll be a bit lower, but measuring 12.1 or 12.2 is not good. And why/how would it measure lower at the Alternator terminal than at the battery?

Then, after that, it's overcharging. Showing 15+ volts is not right, not unless the battery is screwed up or there is a major ground/connection problem someplace. Once it settles down, warm and "normal", it ought to measure around 13.5 or so, at the battery. More than that indicates a problem with the battery or the charging system, somewhere.

The EST and the ignition module do not have a lot of over-voltage compensation built into them and That could be your real problem here. It (high voltage) could also be affecting the ignition coil..

Beyond That issue, a sticky or otherwise messed up EGR valve is a very likely cause of idle speed problems. Disconnect the darn thing and Plug the port (at the plenum) - eliminate it from the equation After making sure that it's really and truly closed. The EGR control solenoid on these years is also a potential problem - they die.

Verify, if you haven't already, that the min idle set procedure has been done, and that the TPS has been re-adjusted to a .50 - .54 volt reading afterwards. It matters, a Lot, that the throttle body min idle stop adjustment is at least somewhere Close to right, and it matters even more that the TPS is showing 0.50-.54 (and I know that the Book says .54 +/- .07, and it'll Run, but higher than .54 is not going to help the idle stay steady..) to the ECM when the throttle is closed. If you want it to start right, and if you want the idle to be nice and steady.

How sloppy are the bushings in the throttle body, or to say it another way - how sloppy is the throttle Shaft? Up to a point it won't make much difference to how steady the idle is, but beyond that point it will make the idle very unstable if the throttle shaft is really slopping around in worn out bushings.

BTW - what do you mean by "replaced the voltage regulator"? There is no voltage regulator other than the electronics built into the alternator... Well, there is a step down as part of the ECM main board, to drop the 12 volt (nominal) supplied to a 5 volt reference level. And tapping or jarring the ECM is Never going to be a good idea, really.

Oh - but tapping on the Block (or heads, or whatever) to verify that the knock sensor is working right might be a good thing. With a timing light on the motor, and the timing connector Connected (so that the ESC system is fully functional), tap (doesn't take much, or shouldn't) on the passenger side exhaust manifold with a smal hammer - the timing should jump when the ECM "sees" the sound.

That's another thing that Might be causing your idle issue, although sort of uncommon. Try simply disconnecting the knock sensor, down on the passenger side of the block. It'll eventually set a code, but not right away, and meanwhile if it was falsely telling the ECM to retard the timing that could be the deal..

As for telling if the harmonic damper has slipped and if the timing marks are off as a result, well.. The only way to do it is to find TDC, #1 cylinder, Mechanically. Accurately. Then see if the timing marks are where they should be. Considering that the damper (if it's original) is 25 or 26 years old now - my money is on the Slipped side. It would not be at all unusual. Set the timing "by ear", and forget about the light and marks. Ignition advance is Not going to be the root cause, nor is it the solution, to an unstable idle.

Almost forgot (OK, Did forget..). Unhook the oil pressure sender (the One wire sender, the Two wire thing is the fuel pump switch) and see if the dash display changes. Then jump that wire (still unhooked from the sender) to Ground and see what the dash does. Assuming that the Dash display goes thru it's range then the sender is bad. If the dash display does Not go from one extreme to the other then there is a problem Other than the sender, like in the wiring or in the dash electronics. The sender is the most common failure, at least. And that circuit can't effect anything else, either. And yes, I know that getting down in there to that sender is a PIA. It's easier with the distributor pulled out of the motor
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Last edited by rons85; 12-17-2010 at 07:00 PM.
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Old 12-17-2010, 06:55 PM
 
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