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Drivability Problem, bring brain inside...

93Rubie

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Jul 19, 2009
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777
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PA
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1993 Ruby Red 40th Anniversary Coupe
Ok, here we go. I got a 94 Corvette LT1 auto I am working on.
No Codes, runs perfect above 140 degrees coolant temperature.
Runs perfect cold too, the problem is under moderate accleration when coolant temps are between 100 and 140 the car backfires through the intake manifold. The car is still in open loop at this point.

When the car goes into closed loop it runs prefect. I verified this with a scan tool today having someone else drive. Again runs perfect cold as well. Under 100 degree water temp.

Under cruise and very light acceleration it does not backfire between 100 and 140 only upon more throttle will it do so.

I saw nothing in all the data I was seeing that would indicate any obvious problems. Except for the TPS it reads .72V closed and 4.10V open. That is off some and I am going to investigate this. Also, as an experiment I unplugged the MAF and ran the car in continuous open loop. It ran fine at all times and temperatures.

Now that being said, this occurs in open loop between 100 and 140 with everything hooked up. What could possibly be operating in open loop at that time but yet when I set a code will not operate because of the code?

I do not suspect any base engine, basic fuel, basic ignition, or other basic systems as a fault because the car runs perfect most of the time and especially in closed loop.

One thought I had is that what if the Secondary Air Pump is running when it should not be, yet with a code it may not operate at all? IDK? Still researching that one. How would I take out the Secondary Air without throwing a code?

Your thoughts, this is an odd one and not something you find everyday. I personally like this kind of stuff and hate it at the same time. This stuff is what separates the men from the boys. Never hurts to get different ideas from others, however, no one can think of everything, hence the post.

Thanks, and bring your brain.
 

boomdriver

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Joined
Dec 18, 2008
Messages
1,886
Location
texas
Corvette
87 z-51
Ok, here we go. I got a 94 Corvette LT1 auto I am working on.
No Codes, runs perfect above 140 degrees coolant temperature.
Runs perfect cold too, the problem is under moderate accleration when coolant temps are between 100 and 140 the car backfires through the intake manifold. The car is still in open loop at this point.

When the car goes into closed loop it runs prefect. I verified this with a scan tool today having someone else drive. Again runs perfect cold as well. Under 100 degree water temp.

Under cruise and very light acceleration it does not backfire between 100 and 140 only upon more throttle will it do so.

I saw nothing in all the data I was seeing that would indicate any obvious problems. Except for the TPS it reads .72V closed and 4.10V open. That is off some and I am going to investigate this. Also, as an experiment I unplugged the MAF and ran the car in continuous open loop. It ran fine at all times and temperatures.

Now that being said, this occurs in open loop between 100 and 140 with everything hooked up. What could possibly be operating in open loop at that time but yet when I set a code will not operate because of the code?

I do not suspect any base engine, basic fuel, basic ignition, or other basic systems as a fault because the car runs perfect most of the time and especially in closed loop.

One thought I had is that what if the Secondary Air Pump is running when it should not be, yet with a code it may not operate at all? IDK? Still researching that one. How would I take out the Secondary Air without throwing a code?

Your thoughts, this is an odd one and not something you find everyday. I personally like this kind of stuff and hate it at the same time. This stuff is what separates the men from the boys. Never hurts to get different ideas from others, however, no one can think of everything, hence the post.

Thanks, and bring your brain.


EGR valve, egr solenoid
Vac leak. intake valve burnt
 

92VetteNY

New member
Joined
Feb 20, 2011
Messages
3
Location
Long Island, NY
Corvette
1992 Black Rose Coupe
Drivability Problem

Ok, here we go. I got a 94 Corvette LT1 auto I am working on.
No Codes, runs perfect above 140 degrees coolant temperature.
Runs perfect cold too, the problem is under moderate accleration when coolant temps are between 100 and 140 the car backfires through the intake manifold. The car is still in open loop at this point.

When the car goes into closed loop it runs prefect. I verified this with a scan tool today having someone else drive. Again runs perfect cold as well. Under 100 degree water temp.

Under cruise and very light acceleration it does not backfire between 100 and 140 only upon more throttle will it do so.

I saw nothing in all the data I was seeing that would indicate any obvious problems. Except for the TPS it reads .72V closed and 4.10V open. That is off some and I am going to investigate this. Also, as an experiment I unplugged the MAF and ran the car in continuous open loop. It ran fine at all times and temperatures.

Now that being said, this occurs in open loop between 100 and 140 with everything hooked up. What could possibly be operating in open loop at that time but yet when I set a code will not operate because of the code?

I do not suspect any base engine, basic fuel, basic ignition, or other basic systems as a fault because the car runs perfect most of the time and especially in closed loop.

One thought I had is that what if the Secondary Air Pump is running when it should not be, yet with a code it may not operate at all? IDK? Still researching that one. How would I take out the Secondary Air without throwing a code?

Your thoughts, this is an odd one and not something you find everyday. I personally like this kind of stuff and hate it at the same time. This stuff is what separates the men from the boys. Never hurts to get different ideas from others, however, no one can think of everything, hence the post.

Thanks, and bring your brain.


93Rubie,

You have given a lot of technical information. Which honestly I do not understand. But this sounds like the same problem I'm having on my 92BlackRose. When I took it out of winter storage last spring I had a similar problem. When it is cold it runs fine. As soon as it hit 190-200 degrees it started to misfire or backfire. I got on it and it got worse. So I limped home. I eventually went thru 2 tanks of gas and still had same problem.
I had new fuel filter and plugs put in(thinking they were fouled) but still had same problem. Haven't driven it since due to lack of funds. So now it's sleeping in the garage.

Thanks :confused
 

LLC5

Well-known member
Joined
Sep 28, 2004
Messages
2,299
Location
Wa.
Corvette
98 black 6spd convert.
Get your TPS in range first, but carbon build up on the intake valves, and injector pattern spray or restricted injectors can cause a lean condition, assuming that your fuel pressure and flow volume are good. Good luck with it. :)
 

93Rubie

Well-known member
Joined
Jul 19, 2009
Messages
777
Location
PA
Corvette
1993 Ruby Red 40th Anniversary Coupe
If it was fuel injection spray pattern or restricted injectors or any other fuel system problem, would I not see this at other times other than my narrow window where the issue presents itself?

I am going to address the TPS issue and spend some more time watching sensor data. Something has got to give me a clue here at some point.

92VetteNY, sounds like yours is happening more constantly and in closed loop. A scan tool would be of help to you. I wish mine would present the issue constantly would make it much easier. However, when issues come and go, it makes things more interesting.

Intermittent problems suck. I'll will keep posted of my progress. I did one change this evening on a hunch and will test drive tomorrow.
 

John Robinson

Gone but not forgotten
Joined
May 3, 2005
Messages
1,555
Location
Muncie, Indiana
Corvette
1993 Polo Green Coupe
From what you say your problem is temperature related. With that in mind check all the sensors etc that talk to the ECM about temp. There is one on the water pump and one in the block. Look carefully for bad wires, broken ground wires, wires pulled out of the connector pins. Also if you trace the ground wires from the water pump sensor and the tps and and a couple of others you will find they all come together in a common ground bundle inside of the wire harness under the valve cover. These grounds need to be cleaned of all corrosion. The temp range you are having a problem in is were the ECM is trying to decide to transfer from open to closed loop and I would guess it is getting a weak signal and going crazy for a short period trying to adjust to bad information.
 

LT4man

Well-known member
Joined
Jun 25, 2003
Messages
2,214
Location
Chicago
Corvette
96 Collectors Edition LT4
Throttle position sensor voltage at closed throttle should be between .45V and .55V.

Make sure your idle speed adjustment has your throttle valves closed while idling. If someone mechanically raised your idle speed (a very big NO-NO!) this would cause your TPS voltage to be too high.

SAVE the WAVE! :w
 

John Robinson

Gone but not forgotten
Joined
May 3, 2005
Messages
1,555
Location
Muncie, Indiana
Corvette
1993 Polo Green Coupe
Here is a little trick I use to test things like the tps. It is so tempting to use a straight pin through the insulation but those wire are very.small and brittle and break easily. I have made up some in line test leads from old wiring harness I found in the junk yard. this way you can connect your meter to the wire without having to puncture the insulation.

Test wires 004.jpg
 

gmjunkie

Well-known member
Joined
Nov 11, 2004
Messages
4,520
Location
Old Fort NC/Vero Beach FL
Corvette
03 Z-51,78 L82 Coupe
The temp range you are having a problem in is were the ECM is trying to decide to transfer from open to closed loop and I would guess it is getting a weak signal and going crazy for a short period trying to adjust to bad information.
That would be my guess too,John!~!!:thumb
That sensor is located on the front of the water pump!:thumb:thumb
And Yes,That sensor just a tad out of range or on it's way out,Can cause the Exact Problem he's having!:thumb:thumb:thumb
 

93Rubie

Well-known member
Joined
Jul 19, 2009
Messages
777
Location
PA
Corvette
1993 Ruby Red 40th Anniversary Coupe
Update, my hunch did not pan out. I have not had time to address the TPS issue yet. Tomorrow is another day.

I drove it work this morning same thing happened at the same time. However, I did notice when I got to work my Long Term Fuel trims where on the rich ride about 140ish for the right bank and 130+ish for the left bank. Also, idle speed in park, engine hot was 1K. Requested idle was 625RPM. IAC was 0. WTF??? that doesn't make sense. In drive, idle speed was about 750 with requested idle was 550 with IAC Zero. Again this does not seem to add up. The more I think about this the more I think I may have a small vacuum leak somewhere. Will investigate tow. as well. I did a quick visual when I got home and saw nothing obvious.

On my way home after work with scan tool hooked up same data as before. However, the car cold the coolant temp was 84 and air temp was about 66. The car did NOT produce the backfiring in the intake at all on the way home. Even in the 100 to 140 range nothing. So it seem this happens more often when outside temps are lower like below 50 degrees. I had light frost outside this morning.

Car does not go into closed loop until 140 indicated on scan data. The backfiring can and does occur at lower temps like 120.

I do not know the complete history of this car, however, from working on it, someone who owned this previously did work on it and they didn't know a piston from their sister's (ahem). This car is a basket case I think the guy paid too much for it. I have done so much work to it and fixed dozens of individual problems. The car keeps getting better and better however.

I apprentice your guys suggestions, I keep thinking of other things from them, so that is very helpful. Now I just need time to investigate. Never have enough time you know.
 

93Rubie

Well-known member
Joined
Jul 19, 2009
Messages
777
Location
PA
Corvette
1993 Ruby Red 40th Anniversary Coupe
Here is a little trick I use to test things like the tps. It is so tempting to use a straight pin through the insulation but those wire are very.small and brittle and break easily. I have made up some in line test leads from old wiring harness I found in the junk yard. this way you can connect your meter to the wire without having to puncture the insulation.

View attachment 3719

Got it covered, back probes. Homemade ones, a little wire, some sewing needles and a soldering gun are all you need.
 

gmjunkie

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Joined
Nov 11, 2004
Messages
4,520
Location
Old Fort NC/Vero Beach FL
Corvette
03 Z-51,78 L82 Coupe
Also, idle speed in park, engine hot was 1K. Requested idle was 625RPM. IAC was 0. WTF??? that doesn't make sense. In drive, idle speed was about 750 with requested idle was 550 with IAC Zero. Again this does not seem to add up. The more I think about this the more I think I may have a small vacuum leak somewhere. Will investigate tow. as well. I did a quick visual when I got home and saw nothing obvious.
I'd say you have a Vacuum Leak,Loose Throttle Shaft (Leaking Air Past) or someone has tried to raise the IDLE by turning the Throttle Plate screw!!My guess is the latter,with 0 IAC count it would have to be a purdy big Vacuum leak!! On 94,95,96,LT1/LT4 the TPS is non-adjustable the ECM does the work ! You should have between .45-.55 V with throttle plates closed. But you may not get it that low unless you set the throttle screw!~!!;shrug

(See Attachments Below)


Throttle Plate Adjustment


Idle Learn.............

 

Attachments

  • Idle Learn 1.jpg
    Idle Learn 1.jpg
    66.9 KB · Views: 77
  • Throttle P Set 94-96 C4.jpg
    Throttle P Set 94-96 C4.jpg
    109.8 KB · Views: 80

John Robinson

Gone but not forgotten
Joined
May 3, 2005
Messages
1,555
Location
Muncie, Indiana
Corvette
1993 Polo Green Coupe
I think Junk is on to something. :BDH but I can tell you that I had a very similar situation with the idle speeds on my car and finally it just died and would not start. It turned out to be the TPS had gone bad and it was not throwing any codes when it did. It took me about 4 weeks to accept the fact that the tps I had put in 6 months earlier was bad. I bought a new one from GM and replaced the after market cheap tps and all was well.
 

93Rubie

Well-known member
Joined
Jul 19, 2009
Messages
777
Location
PA
Corvette
1993 Ruby Red 40th Anniversary Coupe
Sprayed some carb cleaner around today and no change in idle, so I would say no external vaccum leaks. However, its funny you mention the throttle shafts. I pushed down on the lever for the butterflys and the RPM's dropped at least 100RPM very noticeable by ear.

I swapped my TPS off my 93, known good unit. Had same readings on the 94 as the 94's original one did. So not a bad TPS.

I will take a peak at the throttle plate screw and report back. However, it seems that my throttle body is junk considering the air leaking past the shafts.

I should note this car had a air foil on it which I removed with no change.

I also Ohm'ed the injectors for grins and giggles...found nothing out of spec. all around 13ohms.

I checked this, now back to the garage to keep going on this thing....


I'd say you have a Vacuum Leak,Loose Throttle Shaft (Leaking Air Past) or someone has tried to raise the IDLE by turning the Throttle Plate screw!!My guess is the latter,with 0 IAC count it would have to be a purdy big Vacuum leak!! On 94,95,96,LT1/LT4 the TPS is non-adjustable the ECM does the work ! You should have between .45-.55 V with throttle plates closed. But you may not get it that low unless you set the throttle screw!~!!;shrug

(See Attachments Below)


Throttle Plate Adjustment


Idle Learn.............

 

93Rubie

Well-known member
Joined
Jul 19, 2009
Messages
777
Location
PA
Corvette
1993 Ruby Red 40th Anniversary Coupe
SON OF A MONEKY. I have lots of torx bits at home, and some tamper proofs at work. Hence the problem. What size is that T20?
 

gmjunkie

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Joined
Nov 11, 2004
Messages
4,520
Location
Old Fort NC/Vero Beach FL
Corvette
03 Z-51,78 L82 Coupe
SON OF A MONEKY. I have lots of torx bits at home, and some tamper proofs at work. Hence the problem. What size is that T20?
Witch ones,Throttle body screws or Throttle plate screw?? Last throttle plate I adjusted was I think a small Allen,I think 3/32nd or 1/8th. Has the steel plug already been removed out of it?
:confused
 

93Rubie

Well-known member
Joined
Jul 19, 2009
Messages
777
Location
PA
Corvette
1993 Ruby Red 40th Anniversary Coupe
Witch ones,Throttle body screws or Throttle plate screw?? Last throttle plate I adjusted was I think a small Allen,I think 3/32nd or 1/8th. Has the steel plug already been removed out of it?
:confused

The throttle body has a stop screw that seems to adjust both throttle plates position relative to the throttle body. It is on the drivers side of the throttle body. Had to remove the accelerator cable bracket to access. You look inside the hole and I saw the tamperproof torx.
 

gmjunkie

Well-known member
Joined
Nov 11, 2004
Messages
4,520
Location
Old Fort NC/Vero Beach FL
Corvette
03 Z-51,78 L82 Coupe
The throttle body has a stop screw that seems to adjust both throttle plates position relative to the throttle body. It is on the drivers side of the throttle body. Had to remove the accelerator cable bracket to access. You look inside the hole and I saw the tamperproof torx.
Yeah,Thats it,the 92 and 96 I have here still have the steel plug in them and I can't see the screw head!:thumb
The last one I had to set has been awhile,It's something that you don't normally have to do unless someone had messed with it!!;shrug
 

93Rubie

Well-known member
Joined
Jul 19, 2009
Messages
777
Location
PA
Corvette
1993 Ruby Red 40th Anniversary Coupe
Judging my all the other crap I have had to fix on this car and the obvious work of previous owners who shouldn't work on a weed eater much less a corvette. I am willing to bet some tampering took place.
 

93Rubie

Well-known member
Joined
Jul 19, 2009
Messages
777
Location
PA
Corvette
1993 Ruby Red 40th Anniversary Coupe
Update

I adjusted the throttle tonight per instructions. Cold I now have IAC counts. However, the backfiring is still present but not as bad-ish. Still goes away when you give it more throttle opening.

When hot the car does not have any IAC counts, if I turn on the A/C it goes up a ton. Good, but it will flcker 1-4 when going from Park to Drive and so forth then resume Zero. The idle speed is now much closer to desired idle speed. However, it is still about 50-75RPMS above desired idle speed.

If I push on the throttle shaft RPM's drop about the 50-75 or so the desired range. So I evidently have a lot of air getting around the throttle shafts.

My TPS voltage is better at .66-4.10 but still not correct.

I was thinking of backing off the adjustment screw some more but it may not be touching the bracket when I'm done. :confused

I used some propane today and found no change in RPM anything at idle so I think I can rule out any external vacuum leaks.

I am thinking this throttle body is JUNK. However, if replaced would the drivabillity issue go away?:ugh

I also noticed because I test drove it that if I run the car until it is hot like 200 ish then let it sit, I then got in and water temp was about 170, then drove it a short distance and hit the throttle it did the backfiring in the intake. However, I quickly scanned down and saw I was still in open loop, car had not run for more than a minute. So it can backfire at warmer temps just in open loop at low to moderate throttle openings. Above 3K RPM's and it is fine.

I do not know if they changed this for 94 but my FSM 93 says that the grounds for the TPS, ECT, and other various sensors use the ECM for ground. So I could only check the ECM grounds. Which I may do because wilder things have happened.
 

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