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Official Forklift Driver
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Discussion Starter #1 (Edited)
Hi All,

I'm having an intermittent problem of some sort. It's been going on since I got the car back from the hotcam/exhaust/gear install back in Dec. of 2002. We found a busted O2 sensor (getting a code 44), replaced it and the problem went away, until just recently. I did have the FP up to 50psi before I replaced the O2. I though I had it at 47psi for the replaced o2 but when I checked it again weeks later when the SES started up again I was @ 52psi! :surprised I now have it down to 42psi.

What happens is the check engine light will come on and when I let off the gas, instead of gradually decreasing rpm's w/ speed (I have an A4), the rpm's drop down to idle and sometimes low enough to stall the engine. Just the same as if you were driving a manual and when you let off the gas and put the shifter in (N), the rpm's drop instantly. Then when I give it gas again, the engine will stumble for a few seconds, then it drives ok but it stumbles like mad when I try to maintain a constant speed. I checked the code and now I'm getting a 33.

I hooked up a scan tool and found that when the SES light comes on my O2 is reading 100-300mv at idle. Normally it's 300-700mv. I also noticed that when I let off the gas and the idle drops like a rock, so does the injector pulse times, it drops to 0.0ms for a few seconds then it comes back to 1.xms. MAF normally reads 12-13gr/s but when it's acting up it mostly reads 10 and will flash 11 sometimes. Also spark advance at idle went down from 29-39 to staright low 30's high 20's.

I tested the IAC and according to the sheet from MAD, it's supposed to read 40-80ohms, I'm getting 87ohms. I'm thinking that maybe the IAC is the culprit here, but you think maybe the new o2 is damaged due to the high FP (52psi) I was running? Let me know if you need anymore info, I'll have the scan tool for just a few more hours.

Thanks,
-John

*edit, I also need to point out that my IAC counts were around 80-90 at idle when the SES light came on. I have a normal reading od 25-40 when the engine is warm. I got a 72 at cold start-up. Is 80-90 really high or is that # normal?
 

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El Teafive
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10,489 Posts
Do you still have the Cats?
Is the Fuel pressure regulator the original?
What code(s) are you getting?
Did you replace the o2 again?
Where is the o2 located? you said you have headers is the O2 Heated?


My best guess is if you have a Cat and a Non-heated O2 sensor, you may have cooked the Cat Converter
 

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Official Forklift Driver
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7,068 Posts
Discussion Starter #3
-=Jeff=- said:
Do you still have the Cats?
Is the Fuel pressure regulator the original?
What code(s) are you getting?
Did you replace the o2 again?
Where is the o2 located? you said you have headers is the O2 Heated?


My best guess is if you have a Cat and a Non-heated O2 sensor, you may have cooked the Cat Converter
I have a Catco main cat. Precats were cut off the stock front y and the chopped front y is connected to the Hedmans. I have an o2 bung welded on the driver side collector, my year only requires one. I used the stock unheated replacement for the new o2. The FPR is an adjustable that I bought from MAD. I have not replaced the 2nd o2 yet. I would like to get my car on the lift today @ PHP so I can replace the collector gaskets, going with the copper gaskets, they BETTER NOT LEAK!! :D

Both are leaking badly. Could that be it?

I'll check out the cat if I have enough time. :cool:
 

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El Teafive
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10,489 Posts
With the o2 being the the collector, I would suggest a Heated O2.

Also check the Cat, my feeling is that it may be bad due to the non-heated O2 not working completely correct being so far from the Heads
 

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Black 08, A&A TI supercharger. Kooks headers. CCW wheels. Much more
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i didn't think your year had a heated o2?does it?you probably already tried all this but...
whats the reading on the TPS switch?
was the trany cable reset?
the c5 is very sensitive about intake and exhaust leaks,not sure on your set-up.
one thing,when the car is idleing,unplug the IAC,if theres no change in idle,good chance its that.
funny thing about the adjustable fuel regulators is the computer see this as 'high' pressure,so it will adjust the injector duty cycle to compensate(unless your tuned for that).one other thing i have fiqured out while trouble shooting,is 99% of the time if a problem is intermittent,it's usually electrical,but intermittent means once in a while,sounds like your problems are every time you drive.make sure none of the wiring harnesses are near anything hot.
did you ever get the gears and trany shift points adjusted for your gears?you could have a couple things going on at once here.i think you went with 3:45s?that will freak out the stock auto big time.
and if someone else put your motor together you have to wonder if the TPI plenum gaskets are on the wrong side and backwards,and the intake gaskets also.this is pretty common.
sorry,one last thing,sometimes the sensors aren't bad bud the connector that crimps to the connector is bad,thats what happen to Hawaiian Punch.after 3 IACs.hope it makes sense,yesterday was long with the hurricane,plus i worked all night and haven't slept in about 36 hours.:) :)
woops one other thing,i use auto-tap alot on my car,when i was new at it i was always confused about the o2 readings,then it was explaned to me that if your reading is 'rich',it means the car is actually running 'lean'.the computer dumps fuel to cool cats and prevent detonation,so the o2 reading can be little misleading,and that works vise-versa also.i ported the snot out of my MAS and auto-taped,the readings were way rich.that when i learned about the computer dumping the fuel,will retard the timing also.i'm not going to proof read this,good luck,keep posting.:smokin:
 

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Official Forklift Driver
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Discussion Starter #6
Thanks for the input hawaii-5-lo!:cheers:

The stock O2 is unheated. I tested the TPS and that is good. I haven't done the IAC unplug test, yet, thanks for mentioning. ;) I do have an exhaust leak out of both header collector gaskets. The trans has been rebuilt for more power along with adding a Vigilante 2800, but those were done when I put in a new O2. I had the problem before and after. It doesn't happen all the time, I'd say 25% of the time it does this. Sometimes when I put in Park it will clear up if that helps anyone any.

I asked the chip tuner what FP I should start with he said 42. I was high initially b/c I was getting a lean code and wanted to make sure it didn't get too lean. Anyway, I have it at 42 right now and by the reading on the scanner (LTFT 108-118 when the code is set) I could probably drop anther 2psi., which is still higher than the stock setting of 37.5psi. I did the install of all the intake stuff, I'm sure I had all the gaskets in the right spot. I specifically remember asking for a second opinion on the intake gaskets, especially a double check on the rear so I dont get that dredded oil leak.

What does desired Idle mean on the scan tool? I hope that's not what the chip is trying to achieve b/c I have the idle in the 900 range.
 

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Desired idle read in steps,0 to 255 Steps.thats what the auto-tap book says.it also has 'desired idle postition.same range 0 to 255.its a little long to explain,is yours reading steps?or is the 900 rpms?if its rpm,then thats pretty much the "target".the other one tells how far the valve(IAC)is open,example at idle the IAC should read 0 (no control-or bypass,as you increase the throttle the number should go up(remember steps)and this one also ties in with,"Learned idle position."
if i remember,you put a beefy cam in,yah?maybe thats why the tuner set the idle at 900.does it idle good or just o.k.?when just idleing,the IAC is actually closed(not operating).if your scanner reads"IAC learned" it should read 'yes'.it seems like the TPS and IAC might be fighting each other.maybe ask a different tuner(2nd opinion)if your set-up needs a change in the TPS setting.could be as simple as the TPS switch telling the computer its say 1/2 open,and the IAC is seeing more air and telling the computer something different,so the computer just doing the best it can to keep air/fuel/timing in the ballpark.my buddy has my fuel injection books or i could research more on your year car.
one thing i forgot to ask,is the fuel pump o.k.?i think the stock L98 fuel pump can only support 300hp,you may be over that.fuel filter.do you know how to reset the the tranny cable,on the side of the plenum running along side the throttle cable?
 

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Official Forklift Driver
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Discussion Starter #8
Todd, the desired idle reads in rpms. The IAC reads in counts. The scanner is reading 700 rpms desired idle. The idle is not so lumpy at 900 as it is at 700 so I have the idle set at 900rpms +/- 50rpm. Speaking of fuel injection books do you have the "How to undestand and modify...Corvette fuel inj. management" book? That book rocks!!

I wish it was easy as putting it on an auto tap to see whats going on. There used to be a guy local to me that had the program to use w/ my computer but he moved away to Fla. before I could get to him to read my chip. I also didn't get a chance to get the car up on a lift yesterday. :(

I tried to set the code yesterday but could not set it. Sometimes when I try to maintain a certain speed it will set the code but it didn't do it at all. I have 4 things to look at and it's going to take time. The collector gaskets, the IAC harness is broken so I can't snap the plug together, i have to repair that, correct the speedo and drop the fuel pressure. Once I get those straightened out, it should be easier to diagnose the problem. When the code isn't set it drives pretty good. I don't know what the rating is for the fuel pump. I hope I don't need to get a bigger one, I just put a brand new one in there 2 1/2 years ago.


I do have to take the car back in to the trans shop to have it looked at after the speedo correction b/c I get a nasty sound when manual shifting into 3rd at light to medium throttle. I don't want to mess with the TV cable for that reason. It makes the same sound as the starter does when you try to start the car when it's already running.
Thanks again Todd,
-John
 

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Yikes,that sucks when the cars not running right.i'm sure it will be great once it's dialed in.didn't know you had tranny issuse too.the book you mentioned is my favorite book!my f-body friend has it right now,and i gave my c4 books to Hawaiian Punch when i sold my 89'.i'm just tossing out the info i can remember,and between everyone elses advise here,you should be able to fiqure everything out.the scanner is a huge help.there probably is a issue with the IAC connector also if the harness broken.does't take much tugging to disturb the wires were they crimp inside the connector.once thats fixed you should 'hopfully' get different IAC numbers and a smoother running car.that is a good proven combo you have on the motor.hang in there, :smokin:
 

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Cleaner than Stock, Faster than Phock
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Official Forklift Driver
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Discussion Starter #12
Re: mass air flow sensor

jerseyvette said:
I'd ignore the light and possibly buy a mass airflow sensor. The used dealers always have them laying around.
I did that in the spring and it didn't fix the problem. :( Just so everyone knows, Vette2vette has TPI MAF sensors for $50! :cool:

When I moved last month I found a shopping cart worth of parts in storage for my Vette. I have to start a garage sale or something, I have 2 unmodified MAF sensors sitting around collecting dust along with the modified unit on my car now.

LD85, 58 psi out of the stock pump? :surprised :surprised :smokin:
 

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Official Forklift Driver
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Discussion Starter #13 (Edited)
I worked on the car over the weekend and on Monday. I checked the O2 and it was good. I corrected the speedo using an electronic ratio adapter and that helped BIG TIME!! Now the trans is shifting when it's supposed to. All I replaced was the collector gaskets (copper) and the IAC valve. I still have a bit of a collector leak b/c the flanges are warped, but it's not as bad as before when chuncks of the paper gaskets were missing. It acted up on the 2nd start after the IAC replacement and hasn't acted up in the last 20 starts. I noticed also w/ the old IAC in it would act a fool when trying to maintain a certain speed. Now, with the new IAC, it's PERFECT!! :cool: I drove it around today stopping and starting, tying to trigger the code and I can't do it.

I'm guessing my IAC wasn't completely shot, just weak b/c I didn't set a code every time I drove the car. The idle quality and drivability really sucked even when it didn't act up, it always felt like it had a miss. I may have damaged the IAC when I was putting the car back together last Dec. I remember I was in a hurry and I didn't measure the IAC before I put it back on the TB. That had to be it b/c it's only been acting like this since Dec., it was fine before that.

I'm hoping the IAC was it, crossing my fingers! :smokin:

Now that it's running right, I have to say my 87 sounds nice w/ the Dynomax mufflers, my wife woke up her boss at 4am this morning w/ the exhaust :surprised :D , and this baby wants to tear up the pavement :partyon: and not sputter like it did before! :rolleyes: And to think, those 2 races I won that I wrote about in the drag race section was when the car wasn't running right. :D :D :D

Hotcam w/ 1.6 rockers
2800 Vigilante
3.54 gears
Headers
mmmmmmmmmm POWER!! :excited: :excited: :excited: :excited:

Thanks everyone for all the help and ideas!! :cheers:
 

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Glad to hear that the car is running good, too bad the last test and tune at Route 66 was last night (it was a good one, couple of jet dragsters and some Pro Stocks). Oh well, there's always Byron and 41.
 

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:thumbsup: :smokin:
 
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