Corvette 86 auto 50k miles

Hey guys,
I did almost a full tune up on the car and the throttle response still doesn't feel 100% right especially on take off (slight hesitation).

I have recently replaced (under 100miles) spark plugs, MSD leads, oxygen sensor, air filter, IAC, pcv, fuel filter, fuel pump relay, checked and adjusted tps, checked resistance on all injectors, cleaned throttle body from carbon excess, replaced all vacuum hoses.

What else could it be? Anything i have missed?

Car is only throwing egr code.

Thank you in advance

 

is the EGR still installed? did you test the EGR diaphragm with a Vacuum gauge. What about the EGR ports any build up?

 

Yes EGR is still installed but its throwing out a code, could be from a bad temp sesnor or solenoid. I have not tested the EGR diaphragm yet. Would the EGR hurt the throttle response?
Thank you for your response

 

absolutely. If the shaft builds up carbon and the diaphragm shaft is seized in the wrong position. you could lose a lot of power. can you pull up from the bottom of the diaphragm with your fingers. to check it's motion range. at one point it should almost stall the engine. If running. What is the specific CODE.

 

I over looked it thinking it could not be part of the cause
I will have it checked asap.
Throwing code 32
Thanks heaps for your help, much appreciated

 

To improve throttle response when an engine is running normally you can do a few things:

- Advance timing
- Increase idle TPS voltage
- Upgrade to a bigger throttle body

 

your Code 32 Faulty or loose wiring and/or connections at the EGR solenoid. you still should check the EGR diagram. Is your Idle kinda lopy?

 

Yes my idle is slightly lopy on cold start but then it goes normal when it warms up and sits on about 700-800 rpm.
I tried reaching out to the diaphragm with my fingers but it seems stuck in its place, wasnt able to move it up or down. Seems like a bad egr valve

 

If you buy one buy gm. Good luck!

 

I would check out the EGR, but I wouldn't bet that being all of the issue.

Try advancing timing to 8*, L98s tend to like that.

 

Clean and inspect the connectors also then put some dielectric grease to preserve them. Also a second on advancing the timing mine doesn't hesitate when I floor it anymore.

 

Thanks for your help guys
I will try advancing the timing when i get the right tools.

Do you guys also recommend replacing the cap and rotor? Im sure it has never been replaced since manufacturer, it has only done 50k miles though. ..

 

When I got my 85 I took cap rotor apart and it looked fine(140000)
so I put it back together doesn't hurt to inspect it. To advance the timing
just turn off the car unplug(this is important)the brown advance wire and loosen distributor and turn it about 1/8 inch counterclockwise that should do it. Tighten bolt and plug brown wire or advance wire and try it.
No special tools needed.

 

Not sure I would simply advance timing. What exactly it will do, I'm not sure. Advancing timing via the distributor will advance it all over. What parts need it and what parts don't I'm not sure exactly. Regardless, unless you know what it is doing and why, simply advancing timing to make it feel better might be masking a problem. Figure out the problem first then twist the distributor if you must, get it changed in the prom would be a better option.

Not sure what the cutoff is before the ECM considers it out of idle mode. I'd be careful to check up first.

Bigger isn't always better. Your stock throttle body flows more air than the stock engine can use. In fact, I knew someone who did and while the butt dyno showed gains, the track showed he had gone slower with the big TB and faster with the stock.

IIRC my stock 91 was reasonably decent off the line. Certainly not like what it is now but for a stock unit, it was good. If his does not perform well, perhaps the thing to do is check maintenance and do overdue stuff. I would add that most people don't send injectors off for cleaning and testing but it does have some effect on the fuel so I'd take them off for cleaning and testing. Cap and rotor, if they are that old, I'd get new one. Check spark for blue flame. Interrogate ECM for air leak. Clean TB, IAC housing and IAC. Get everything running to spec before you start adding new items to confuse things.

 

 

Some guy who called himself a mechanic set the timing to 19 degrees because he didn't disconnect the advance wire and the car ran with "no problems" either. I am not sure what they mean by "no problems". Does that mean the ECM compensated for it but you weren't able to realize it because you didn't datalog or does it mean there is no compensation by the ECM? IF it is the former, "no problems" means "nothing I can feel in the butt dyno. If it is the latter, it means there really are no issues the ECM has to work around. Two totally different scenarios.

Can it cause you to WOT the throttle but there isn't enough voltage to inform the ECM? I'm not sure. I know you have a 5V reference but not sure what the max voltage output is. I'd check with a scanner and engine off at the very least. My 91 takes whatever voltage it starts up with as idle. I still set it to 0.54V but I forget what the max TPS voltage is but I know it can't exceed 5 and I am not sure what the TPS arm does if you extend it past it's point when you WOT. I'd check that for sure.

Being that I am not a programmer, IDK but I thought that at WOT, it ignores the O2 sensor and goes off programming so how does that affect the way the ECM executes the program?

I do know that on my F-body, based on the SWAG of the butt dyno, after I put on the Superram Intakes and Tri-Y headers. Same butt dyno felt it was definitely more responsive after Lingenfelter tuned it. IMO, safest and best bang for the buck in the long run is to get it done via tuning instead of hacking and forcing the ECM to compensate.

 

Daniel is right about all of his points. Advancing the timing "globally" by turning the distributor DOES have an overall positive effect on many engines. Why? B/c the OEM specs were more conservative than they need to be, and they are that way, "globally". The potential for gains here depends on the engine and calibration, but on some engines (like the CFI, for example), the gains are pretty profound for a simple calibration adjustment.


Covered above. Here is one example; my '83 CFI TA. It's stock 305/CFI was rated at 170 hp. That is good for mid-high 15's/high 80's, in a ~3300 lb car. I put on Edelbrock "headers", installed a 160 stat (hoping to gain greater detonation resistance), advanced the timing as much as it would take w/o detonation, removed the mechanical fan, and cranked up my fuel pressure w/the stock regulator. With those changes, car went 14.5/95mph, which takes about 220-230hp to accomplish.



Youre car doesn't "Take whatever voltage it starts up with as idle". It's looking for ~.5v About. Doesn't have to be a specific, exact value. There is an acceptable range just as there is at the top end. Voltage won't exceed 5v at WOT no matter how you set the TPS. First, it only has 5v for reference, second, typically, a properly set TPS ends up some where just above 4v at WOT. There isn't enough range in the adjustment to hit over 5v....even if you had a greater than 5v reference, which you don't.


The ECM is looking at inputs from sensors. If you install a larger throttle body, at a given throttle opening, there will be more airflow than a stock TB, as Daniel correctly stated. We are talking part throttle here, but part throttle, or WOT, the ECM is looking at MAF or MAP values and RPM for air flow, more so than O2. O2 os for feedback/trimming...not course A/F adjustments.

 

I don't know what the program does but that was what I was told by the tuner. I think he mentioned a range, now that you mention it but I wasn't sure. I understand you cannot go past 5V reference but I also know the TPS arm does have a certain amount of "sweep". So at the throttle's physical WOT, what does it do to the TPS if it runs out of sweep BEFORE you hit throttle WOT? Or will it? IOW, what happens if the sweep ends before your throttle blades are fully open and you press more?

So how did we see my friend's C4 going slower consistently with a 1000 cfm throttle body? I can't say if it affected the response but I do remember us bolting it up, running, unbolting it and replacing stock TB about 2 or 3 times each and each time, it ran slower with the bigger TB. If you are correct, doesn't that mean you get better response at the cost of a better overall performance at the track?

 

The TPS design (it's mounting ears, and internal construction) won't allow you to mount it in such a way that you can over extend the internal's "sweep".


I can't comment on your friends car since I wasn't there, didn't do the work to it, and didn't see what was going on there. A bigger TB shouldn't have negatively affected WOT performance, although it may not have been any help, either, depending on the engines requirements. A 1k CFM TB should have most definitely made the "throttle tip in" or off idle throttle response, much more pronounced.


.

 

I am sure that my stock TPS has 2 screw holes. If you want adjustment, get an earlier model or **** it yourself. That was why the tuner told me that it takes whatever voltage as 0% throttle, IIRC. That was why I was wondering what happens if you rotate it till WOT runs out of rotation but if you say it won't, fair enough.

I don't think we went down that road. Just measured quarter mile times with and without back to back. Why it ended up slower, I can only theorize that it somehow affected air flow. Could it just be better only because the rest of the intakes were set up as was the ECM?