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Drivability Problem, bring brain inside...

93Rubie

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777
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1993 Ruby Red 40th Anniversary Coupe
Another Thought?

So the car runs just fine all the time with the MAF unplugged, obviously causing a code and open loop.

What is the difference in operation if the MAF is unplugged with codes in open loop versus no codes and open loop?

For example: Does the car run richer if we have codes in open loop versus just open loop?

My guess is yes, because idle speed is increased and the cooling fans are on. Then again that is a guess.

Food for thought.
 

gmjunkie

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Nov 11, 2004
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Old Fort NC/Vero Beach FL
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03 Z-51,78 L82 Coupe
Plug off the Power Brake Booster and see if the IAC counts go up!!
If so there is probly vacuum leaking in the booster,Check out the Cruse control hoses good too!~!!(Ex-Specially under the dash!)

:upthumbs
 

John Robinson

Gone but not forgotten
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May 3, 2005
Messages
1,555
Location
Muncie, Indiana
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1993 Polo Green Coupe
Check the vacuum check valve under the valve cover on passenger side. It has two vacuum lines coming from it one for the cruise and one for the heater. These tend to have the connectors brake or the hoses go bad from the heat and time.
 

93Rubie

Well-known member
Joined
Jul 19, 2009
Messages
777
Location
PA
Corvette
1993 Ruby Red 40th Anniversary Coupe
I replaced the vacuum check valve last year that fixed the inop. mode doors.

I went for the jugular and plugged off the vacuum brake booster at the manifold=no change in IAC count.

Plugged the main vacuum feed line for the inside of the car/cruise control=no change in IAC count.

The only thing I have found for sure it that the throttle body is junk. It idles at about 700RPM hot, with 625 desired. If I push on the throttle blades to make them close more I can feel them move some and RPM's drop about 50RPM.

Here is the thing do I go with a used throttle body and risk getting another junk one or find a new one?

Any other ideas? I'm out of them, other than the throttle body.

Unless they were messing around with the tune on the car? Which is a possibility. I would have really no way of knowing this or fixing it for that matter.
 

LLC5

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Sep 28, 2004
Messages
2,299
Location
Wa.
Corvette
98 black 6spd convert.
I replaced the vacuum check valve last year that fixed the inop. mode doors.

I went for the jugular and plugged off the vacuum brake booster at the manifold=no change in IAC count.

Plugged the main vacuum feed line for the inside of the car/cruise control=no change in IAC count.

The only thing I have found for sure it that the throttle body is junk. It idles at about 700RPM hot, with 625 desired. If I push on the throttle blades to make them close more I can feel them move some and RPM's drop about 50RPM.



Here is the thing do I go with a used throttle body and risk getting another junk one or find a new one?

Any other ideas? I'm out of them, other than the throttle body.

Unless they were messing around with the tune on the car? Which is a possibility. I would have really no way of knowing this or fixing it for that matter.



Make sure that the throttle body isn't carboned up. ;)
 

helphos

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Joined
Dec 17, 2009
Messages
68
Location
Maryland
Corvette
1993 Polo Green Vert
backfiring-

:bang I'm just lurking, my 93 makes a slapping noise on acceleration, and while it sounds like a 'physical' noise, I'm thinking it could be something like what's going on here- backfire or knock.
I'm gonna start looking at my vacuum hoses this weekend.
Carry on!
:eyerole
 

93Rubie

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Jul 19, 2009
Messages
777
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PA
Corvette
1993 Ruby Red 40th Anniversary Coupe
Update

Throttle body is clean, I cleaned it last year, still fine.

I got the IAC counts in line. I compared the throttle body to my 93 and found the throttle stop was bent, unbent it and reset the set screw according to specs. I now have IAC counts and it has complete control over idle.

I test drove the car again tonight watching the open/closed loop PID. If I start it up, even hot, and immediately take off in open loop (it stays in open for a short while 30secs to 1 minute), it will backfire/miss/general lack of power. However, I got it to do this multiple times and while monitoring the closed/open loop PID. When it goes from open to closed it is like flipping a light switch.

Running like crap to running perfect in a split second. Weird, huh? :ugh

I have no ideas at this point and any legit ones are welcome, short of starting to replace stuff I have no idea. All the data I am seeing is in specs at this time. However, I want to reemphasize this idea I had from an earlier post, that no one addressed but may be the key to this.
See below...

Another Thought?
So the car runs just fine all the time with the MAF unplugged, obviously causing a code and open loop.

What is the difference in operation if the MAF is unplugged with codes in open loop versus no codes and open loop?

For example: Does the car run richer if we have codes in open loop versus just open loop?

My guess is yes, because idle speed is increased and the cooling fans are on. Then again that is a guess.

Food for thought


I am beginning to wonder if someone "tuned" this PCM and judging by the way the screwed up the rest of the car, may have screwed up the PCM, trying to "tune" it. :confused
 

John Robinson

Gone but not forgotten
Joined
May 3, 2005
Messages
1,555
Location
Muncie, Indiana
Corvette
1993 Polo Green Coupe
What is the difference in operation if the MAF is unplugged with codes in open loop versus no codes and open loop?

The difference is that the MAF ground is not connected to the ECM therefore were it's ground wire and the one from the TPS and the one from the Water pump temp sensor and the one from the air cleaner and possible the one from the ABS all come together. So when you unplug the MAF you are in effect removing the ground wire signal to the ECM and ECM is a ground sensitive reader. by that it reads the voltage from the sensor that is going to ground and adjust the fuel trim accordingly (This is my simple explanation) If the ECM is not getting a good ground signal from a sensor it will try to reestablish the number it wants by adjusting the fuel trim. When you disconnect the MAF the ECM then makes changes which may be to enrich in the fuel mixture which would make the car run better. I know I sound like a broken record about the need to clean the ground bundle that all these wires go to but I spent over 5 months last fall and early this year before I cleaned them and my poor performance and bad gas mileage improved along with the car idling again. And by the way I bought a new MAF and it didn't change a thing only fixing the ground connection did.
 

93Rubie

Well-known member
Joined
Jul 19, 2009
Messages
777
Location
PA
Corvette
1993 Ruby Red 40th Anniversary Coupe
I rechecked that particular ground connection and it was ok, cleaned it up some and tightened just to be sure.

On a hunch, I pulled the EGR and it was yucky. So I cleaned it up a lot and put it back in place. Left the vacuum line disconnected and unhooked the A.I.R. so it would run but not blow into the exhuast and test drove. I still have the same thing occurring.

However, it seems that when everything is hooked up it does backfire with less throttle opening and seems like it takes more with the A.I.R. unhooked and the EGR unhooked. IDK? Maybe it is just me.

I even tried unplugging the fuel pressure regulator and nothing changed.

I am out of ideas on this one....

How much you want to bet a lot of people would have thrown a lot of money at this thing to get it right and still not fixed it?

I can tell you what isn't wrong with the car easier than I can what is wrong with the car. Sheesh...up SH** creek with out a paddle.:confused:confused:confused
 

boomdriver

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Joined
Dec 18, 2008
Messages
1,886
Location
texas
Corvette
87 z-51
I rechecked that particular ground connection and it was ok, cleaned it up some and tightened just to be sure.

On a hunch, I pulled the EGR and it was yucky. So I cleaned it up a lot and put it back in place. Left the vacuum line disconnected and unhooked the A.I.R. so it would run but not blow into the exhuast and test drove. I still have the same thing occurring.

However, it seems that when everything is hooked up it does backfire with less throttle opening and seems like it takes more with the A.I.R. unhooked and the EGR unhooked. IDK? Maybe it is just me.

I even tried unplugging the fuel pressure regulator and nothing changed.

I am out of ideas on this one....

How much you want to bet a lot of people would have thrown a lot of money at this thing to get it right and still not fixed it?

I can tell you what isn't wrong with the car easier than I can what is wrong with the car. Sheesh...up SH** creek with out a paddle.:confused:confused:confused

Time to go to the ecm. Can you "see" what the open loop data is vs the closed loop with sensor input? Does anyone have a record or print out of what the data should be from memory in open loop? As in, what the factory tune data was vs whats there now? Could the ECMs processor have been damaged? Thats easy enough to do.

You may be the victim of a reflash gone wrong...people screw with the tune, altering things and never slow down to record what was changed. Heck, I've got 3 proms, all different program, and I usually can't tell you which is which except for the temp when the fans come on or when its 'shifts' to closed loop....No tellin what the PO tried to do to yours.
 

gmjunkie

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Nov 11, 2004
Messages
4,521
Location
Old Fort NC/Vero Beach FL
Corvette
03 Z-51,78 L82 Coupe
Back in July,August,September and October of 08 (Toms94) Aka Toms01 had a problem with what started out as a small hesitation or sort of a stumble at idle!
Wound up being a Bad Cap on a NEW GM OPTI!~!It was a real Joint effort by several Knowledgeable C4 guys!~!(Probly More Beer than Joints!~!!)

:D:D:D

Here maybe this will shed some light!!:thumb:thumb:thumb

http://www.corvetteactioncenter.com/forums/c4-technical-performance/106269-sort-stumble-idle.html

Probly the longest C4 Tech thread in CAC History,255 posts!~!!
 

boomdriver

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Dec 18, 2008
Messages
1,886
Location
texas
Corvette
87 z-51
Back in July,August,September and October of 08 (Toms94) Aka Toms01 had a problem with what started out as a small hesitation or sort of a stumble at idel!
Wound up being a Bad Cap on a NEW GM OPTI!~!It was a real Joint effort by several Knowledgeable C4 guys!~!(Probly More Beer than Joints!~!!)

:D:D:D

Here maybe this will shed some light!!:thumb:thumb:thumb

http://www.corvetteactioncenter.com/forums/c4-technical-performance/106269-sort-stumble-idle.html


Good point !:beer

Assuming that "new" is good, is worse than assuming that old is bad.
In either case, Prove it !


I've been stung good a few times by 'bad' new parts.
 

93Rubie

Well-known member
Joined
Jul 19, 2009
Messages
777
Location
PA
Corvette
1993 Ruby Red 40th Anniversary Coupe
Good point !:beer

Assuming that "new" is good, is worse than assuming that old is bad.
In either case, Prove it !


I've been stung good a few times by 'bad' new parts.

Count me in on that one. I diagnosed a no start on a Nissan Xterra the TCM (transmission control module) kept blowing its main power fuse. So I put a new valve body,TCM inside the body, $1000, and it would start but I had no CAN communication to the TCM. Yet I had comm. on the K-line. The K-line on a Nissan is for the Consult 2 (scan tool) only. So I could talk to the TCM but none of the other modules could talk to it. Vehicle started and drove but shifted bad and lots of dash lights. I wound up putting another "brand" new TCM in it and that fixed it. After hours of diag. and on the line with Techline. I have stumped those guys before but that is another story and a lengthy one.

I will browse that thread, GMJunkie. Thanks.

It is so wired that it can backfire in open loop and as it is doing this I can observe the car go into closed loop and it runs perfect. :ugh Like flipping a light switch.

Boomdriver, I am beginning to think someone may have done just that. Given this cars history, it is possible. Unfortunately, I have no way to see into the ECM. I have only a glorified code reader of a scan tool and it gives me lots of live sensor data and such but nothing like you allude to.

Correct me if I am wrong but the 94-96 PCM's could be re flashed on the fly-no PROM's (unlike my 93) so if I could find someone with the setup to look into this ECM or even someone with a Tech 1 that could reprogram the PCM to stock, this could be done. Right???
 

93Rubie

Well-known member
Joined
Jul 19, 2009
Messages
777
Location
PA
Corvette
1993 Ruby Red 40th Anniversary Coupe
I reviewed that thread Junk, and that is so similar except it is reversed this 94 backfires in open loop not closed like Toms. Go figure. At this point anything could be a problem. Except for the stuff I have looked at. I wish I had a 94 FSM, some of the stuff in my 93 FSM is similar but not the same.
 

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