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94 LT1 Automatic Mis Fire & Knock, Please READ carefully, lots of info.

93Rubie

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Got kinda of a strange issue on my buddies 94. At idle it has a definite miss, maybe 1 cylinder a most. Checked spark off coil with inline checker=constant spark no dropping out. Checked the easy to get too #4 plug wire=steady and even spark.
Running down the road, runs ok at a cruise, WOT, lighter loads. However, if you gently apply throttle as to not get it out of overdrive but still increase the load the car starts to knock, hard metallic knocks. Definlety "pinging" in the old fashioned detention sense. The knock sensors go nuts and pulls 20 degrees right now. However, it is a on/off switch it is either pinging or not. The knock counts go from 1 or 2 to a bunch really quick and then back, when I let off or force a downshift.

For some reason I cannot see High Resolution signal on my scan tool I can on my 93 but not his 94. IDK? Anyhow I can see the low resolution signal and it kinda bounces around at idle. With a warm idle it can bounce around from about 22m/sec to 24/25m/sec. Never noticed my 93 doing this. I did back probe the Opti-Spark connection and at idle, warm low resolution bounces from about .9V to 1.2V. High is even at about 2.4V. Not sure if this is normal or not FSM doesn't really give me the info I want on this one.

Idle timing on scan tool reads about 20 degrees cold/17 or so warm. That seems to be ok.

No CODES in ECM. I did have a 54 and 41 in the CCM as history codes.

Today, after messing with some other things I was spraying down the wiring, just checking, and noticed the number 5 cylinder plug wire seemed to be moving?!?! Sure as pie, the #5 plug was loose. Tightened it up, still had the miss at idle. SOB!!! Plug appeared to have been firing,not wet or fouled. (I pulled it.)

FYI, this car is in sad shape, previous owners ran it hard and put it away wet. The current owner is not much better with it. I see the car when it starts to run not just bad but REALLY bad. Needless to say, its a basket case, you fix one issue, be prepared to fix some more. That and others have really boogered this thing but with backyard HACK fixes, its a nightmare.

Anyone have ideas? My current thoughts are these:
Check all plugs on bank 1 (bank 1 has larger pulse width than bank 2) make sure they all have spark/tight. Then check bank 2 for the same. Pending that is ok, why is the opti-spark low resolution readings jumping around everywhere, just because of the miss, is my guess??? Them moving like that strikes me as odd.

Oh, I did check grounds, tight and reasonably clean.

As usual this started as a no start, died in the rain on him event. :eyerole Turned out battery was just shot. New(er) battery it fired right up but missing.
 

John Robinson

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Have you checked the wire connector on top of the opti for corrosion or water. It also could have a bad ground wire. If as you say this is a abused car my thoughts lean towards bad or broken wires from the TPS, IAC, Water temp gauge etc. They all go into a common ground if you follow them back into the wireing harness. That common ground often is wrapped in tape and will be corroded. Finally you might try fireing it up in the dark and looking for a plug wire that is arking to ground.
 

93Rubie

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Have you checked the wire connector on top of the opti for corrosion or water. It also could have a bad ground wire. If as you say this is a abused car my thoughts lean towards bad or broken wires from the TPS, IAC, Water temp gauge etc. They all go into a common ground if you follow them back into the wireing harness. That common ground often is wrapped in tape and will be corroded. Finally you might try fireing it up in the dark and looking for a plug wire that is arking to ground.

I will recheck that ground, it ohm tested fine to the - batt. terminal but weirder things have happened.
I did fire it last night, no arching, even with water spray, that is how I found the loose #5 plug. I pulled that plug and it appeared that it is firing and no cracked insulator or anything odd.
 

John Robinson

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I just remembered I had something like this after I changed plugs on my car. It turned out that one of the plugs had a crack in the porcelian and it was shorting out on the head were I could not see it. So if the plugs were changed by the previuos owner and he did not use care in putting them in he may have broken that porcelian shield. Easy to do if you get the socket at an angle while putting them in.
 

tmkassin

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First thing I would do is replace the plugs and wires, maybe even the cap on the opti. One bad wire can cause all kinds of havoc.
 

toptechx6

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Try unplugging each injector in turn until you locate the weak cylinder, way easier than disconnecting a plug wire. 94's are sequential which makes the injector method more accurate than the earlier batch fire LT'1s when it comes to isolating a misfire. Once you determine the dead cylinder then figure out if it is lacking spark, fuel or compression.
 

Hib Halverson

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A 94 has partial OBD2 capability. When the OBD2 part of that sets a code it does not turn the MIL on. I'm not sure to what extent that "partial OBD2" goes, but if it includes misfire detection, I'd love to see if there are any misfire codes set.

If your tester will read the OBD2 part of a 94/95 LT1's ECM, check that and see what's there.

Now, in one sentance you say "hard metallic knock" and in another you say pinging. Is it possible the engine has mechanical damage which makes a loud knock which is not detonation but is loud enough to be false knock? An example would be a cracked or broken piston skirt.

While it is possible this problem stems from engine controls, ie: fuel/spark/etc, don't ignore engine mechanical issues.
 

toptechx6

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A 94 has partial OBD2 capability. When the OBD2 part of that sets a code it does not turn the MIL on. I'm not sure to what extent that "partial OBD2" goes, but if it includes misfire detection, I'd love to see if there are any misfire codes set.

If your tester will read the OBD2 part of a 94/95 LT1's ECM, check that and see what's there.

Now, in one sentance you say "hard metallic knock" and in another you say pinging. Is it possible the engine has mechanical damage which makes a loud knock which is not detonation but is loud enough to be false knock? An example would be a cracked or broken piston skirt.

While it is possible this problem stems from engine controls, ie: fuel/spark/etc, don't ignore engine mechanical issues.

94's do have some OBDII functionality, but misfire detection requires a crank position sensor that did not get installed until 96. Checking for mechanical problems would would definitely be high on my list of things to check based on the knock description.
 

Hib Halverson

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94's do have some OBDII functionality, but misfire detection requires a crank position sensor that did not get installed until 96. Checking for mechanical problems would would definitely be high on my list of things to check based on the knock description.

Ooops
So much or the misfire detection idea.
 

John Robinson

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Any votes for a broken rocker or bent push rod to a rocker on the miss. A bent push rod I would think would relax the hydralic lifter and then it would make a noise when it takes up the slack. An over rev could do that kind of damage.
 

93Rubie

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Update

To clarify the hard metallic knock is detention/pre-ignition/pinging. Whatever you want to call it.

I tested ALL plug wires for spark to plugs all have strong consistent spark. I found another loose plug #1 this time. Still has a miss.
The miss definitely gets worse as it gets warmer. Was ok when dead cold start. I removed plugs 1 and 5 (the loose ones) NO issues seen with them. This car ran fine 2 years ago when he bought the car, he has not put plugs in it since. So if they where not broke then, should not be now. I'm not sure when the car started this missing and ignition knock under load.

For grins and giggles I ohm tested all injectors all in spec. and all are pulsing on and off.

Rechecked and cleaned my grounds no change.

At this point I see no reason the miss is coming from ignition system whatsoever. It is working as intended. As long as the timing is not off and I do NOT suspect it is given my good scan data readings. Note:The low resolution signal is steady when its NOT misfiring it goes nuts the more it misfires.

I'm getting my fuel pressure tester and bringing it home tow.
On that note once in closed loop fuel trims cycle rich/lean according to scan tool.
Also, engine vacuum is around 20 inches.
No obvious engine mechanical issues, no noises.
I even tried running the car in open loop with the MAF unplugged same miss.

I keep getting Code 41 in the CCM as history which is loss of ECM serial data. Not sure why. All plugs/connections check ok at ECM.
I even tried tapping on the ECM as the car was running no obvious change. It felt cold too.

I may have to explore engine mechanical issues if the fuel pressure is good and I suspect it is.
I may do a power balance test as suggested, good idea. I did note as I was playing with plug wires all manifold ports felt equally warm.
 
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93Rubie

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Here is a thought I just had:
What if there is water in the fuel?
It has been rainy here as of late. If the cap is not sealing well....it has only the stock drain in the boot.
That could certainly cause a miss, not sure if that would cause a knock but it suppose if it dropped the octane rating enough of the fuel. It certainly could.

I could get some out tow. while I'm testing fuel pressure, add some water, and see if it separates or not.
 

gmjunkie

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I keep getting Code 41 in the CCM as history which is loss of ECM serial data. Not sure why. All plugs/connections check ok at ECM.
I even tried tapping on the ECM as the car was running no obvious change. It felt cold too.
Check the main harness back behind the engine,there bad to get chaffed through the insulation behind the intake!:thumb
 

93Rubie

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@Junkie, I should have mentioned it is EXTREMELY intermittent that CCM code 41. I only ever found it in history. Never current and sometimes it is there other times not. I did check the harness no obvious rubbing or chaffing.

Update:
Fuel pressure in specs at 42psi key on, pump on. Key on, pump off @ 38psi. Engine running, 38psi at idle, vacuum line off 45psi at idle. Sounds ok to me.

I did pull a fuel sample not sure how old this stuff is but I'm not sure my 1941 Farmall H would run on this stuff, very stale and hardly has any "gas" smell to it. I did put in a bit of water in my sample well see how much of it separates out. Early indicates seem it is ok, water content wise. Ethanol absorbs water last I checked.

Thinking it might be wise to drain the tank, of 10+ gallons and try some fresh 93 octane.
Fixing neglected cars really stinks on ice...:eyerole
 

John Robinson

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Gas now that requires the old Sherlock Holmes hat and curved pipe. How long has the gas been in the car? Was it bought at a BP station. (they have been in the news recently with a recall for bad gas) If the car was not being driven was the tank full or half full or nearly empty? Was the car exposed to lot of moisture for a long time without being run?(Space in tank becomes a collector for moisture build up over time). If it is determined to be bad gas instead of pumping out and haveing a problem with disposal why not add dry gas and octane booster to the tank and top it off with good premuim gas. Then go and drive it until it is down a quarter of a tank and refill. My expereince with bad gas is that it usually runs through the engine preatty quick. I filled up once on a trip at one pump with premuim and my son filled his car with regular. I normally would have to fill twice to his once on a road trip but this time in 100 miles he was on reserve and could not run over 60 miles per hour. A fresh refill and within a half mile his car took off and ran fine without any more problems.
 

93Rubie

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1993 Ruby Red 40th Anniversary Coupe
Gas now that requires the old Sherlock Holmes hat and curved pipe. How long has the gas been in the car? Was it bought at a BP station. (they have been in the news recently with a recall for bad gas) If the car was not being driven was the tank full or half full or nearly empty? Was the car exposed to lot of moisture for a long time without being run?(Space in tank becomes a collector for moisture build up over time). If it is determined to be bad gas instead of pumping out and haveing a problem with disposal why not add dry gas and octane booster to the tank and top it off with good premuim gas. Then go and drive it until it is down a quarter of a tank and refill. My expereince with bad gas is that it usually runs through the engine preatty quick. I filled up once on a trip at one pump with premuim and my son filled his car with regular. I normally would have to fill twice to his once on a road trip but this time in 100 miles he was on reserve and could not run over 60 miles per hour. A fresh refill and within a half mile his car took off and ran fine without any more problems.

I have those same questions, I made a phone call to my buddy. We'll see.

I thought about adding dry gas and some octane booster, but the only booster that works is the NOS stuff. Mr. Halverson as I recall, did a article on octane boosters a while back.

Probably best off to dump it in my farmall. Poor tractor....:ugh. Better yet I'll give it back to him in cans and he can put in his tractor. :L

There is a BP local to him....:ugh. I tried BP in my 93 once, or twice. It did NOT run well with it, fuel mileage less, I dropped 2 MPH, on the top end of the 1/4 mile. It seems to like Sunoco, does good with Exxon too. Those are my only local options unless I want Kwik Fill or Sheetz, both of them have crappy fuel.
 

Schrade

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That and others have really boogered this thing but with backyard HACK fixes, its a nightmare.

That's the end of the thread. You can't diagnose a computer controlled car, after it's been hacked. PERIOD.

You CAN start restoration of each 'system', and each 'circuit', AFTER FSM purchase.

"Lots of info" ?
 
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93Rubie

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Trust me it has been fun undoing the wrongs...to this Corvette. Lots of wiring repair and diagnosis. I do not understand why people hack cars and don't take care of things. Boggles my mind, then again I had (have) to work for what I have.
 

roger longman

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That's the end of the thread. You can't diagnose a computer controlled car, after it's been hacked. PERIOD.

You CAN start restoration of each 'system', and each 'circuit', AFTER FSM purchase.

"Lots of info" ?


Hi Schrade, Good to see you back.......Roger.
 

93Rubie

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1993 Ruby Red 40th Anniversary Coupe
Update

Big time update.
I did a little experimentation. The fuel as noted did not smell too good. I got some Starton Fuel Enzyme treatment. The car does not run any better but the fuel smells and looks like gasoline again. Not fresh stuff, but much better. So interesting on that one. Owner called me back, said the fuel was newer. Not sure where he got it. :eyerole

Took the car on a highway run to check some data out. At idle dead miss under load in Drive. Runs down the road just fine while cruising. Still (detention) knocks when you put it under load.

One thing I did notice at cruise fuel inj. pulse is about the same on both banks. At idle bank 1 has about .6 m/s more than bank 2. My long term fuel trims are bank 138 bank 2 118. That got me thinking I could hear them clicking but are they ACTUALLY working.

Unplugged IAC (which oddly had no effect on idle or when I turn on the A/C as far as Idle RPM went, odd but probably not related given other problems) then proceeded to unplug bank 1 fuel injectors one by one.

1 dropped RPM, 3, ok, 5, ok, 7 did NOT drop ANY RPM, no difference in RPM or sound. Lacking noid lights, I improvised my test light. Back probing to the ground side of the injector and the test light on + batt. I got faint pulses. Tried on #5, did same thing, so my ECM is pulsing #7. I can hear it click as well. Fuel injector itself ohms at 13 hot. Checked circuit from #7 ground side to ECM=good.

My thought is that #7 injector is 1: being commanded to pulse 2: is turning on and off, (I was able to KOEO, pulse the injector manual to ground with jumper, could hear fuel flow) 3: Might be plugged up or not opening enough to get adequate fuel at idle into #7 cylinder, hence miss. Better at cruising speeds as more intake manifold fuel saturation.

Now if I had a GM Injector Pulse tool I would know 100% along with my fuel pressure gauge if it was a injector flow issue.
I don't have that tool, so barring any other odd situation I think I got a bad/plugged fuel injector.

Agree or disagree?

FYI, pulled both valve covers to inspect springs, rockers,etc...found nothing obviously wrong. All seemed to be working as designed.

I may do a leakdown test on #7 just to be safe as far as engine mechanical goes.
 

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