Bugly87

Hello all,
1987 Corvette Coupe auto. 56000 miles. I have had the car for the past 3.5 years and don't ever drive fast but I wanted to see what power the car has. Runs and drives fine..in drive on the highway I made 70 mph at 3000 RPM. Shifted to overdrive and got to 83 mph at 2300 RPM. Got on the accelerator all the way and the engine revved to 3300 RPM but the car did not go any faster. Stayed at 83 mph. For lack of better words it felt as though there is a governor on the car preventing it from going faster. I noticed a stumble in the motor at this time. I got as fast as 91 mph if I gradually accelerate but stops as soon as I floor it. The transmission kicks down like I believe it should but does not go any faster. New plugs, wires, cap and rotor 2 years ago...rebuilt Bosch injectors and FPR from Jon at FIC last October...and a new complete fuel pump assembly last month. My mechanic told me that I need a EGR valve so they blocked it for now. It runs better but no power at high speeds.

Any advice or direction would be appreciated..

Kevin

1984Z51auto

I've got questions more than an answers at this point: is your fuel pressure holding at high speeds? Sounds like you might be starving for fuel. Is the throttle opening all the way? Sounds odd for a well tuned L98 to run out of go at such a relatively low speed.

Bugly87

Thanks for the quick reply..I am only somewhat versed in this car..although I did do the injector / FPR swap last Fall. With the new pump and assembly I can only assume that it is holding fuel pressure at high speeds..Mechanic verified that it is holding pressure. Is there a simple way to check to see if the throttle is opening all the way..I mean is it as simple as taking the throttle body hose off and manually holding the throttle open? Should the flap be evenly horizontal? The fuel filter was also changed 3 years ago. To be honest I never tried to drive it fast since I got it..so I don't know how long this has been an issue.. Go figure..a Corvette owner who never tried to drive it fast..lol I got 105 mph in a Fiero V6 so I know this should go way faster. This is my second C-4. I also had a 1980 C-3 coupe.

Thanks again for the reply..I will check the throttle position as soon as I can. I will also check the fuel pressure at higher rpms, too

Kevin

wydopnthrtl

Since you mention a stumble.. I suspect a lack of fuel.
Could be a plugged fuel filter, a broken fitting, a loose fitting (both in the tank), a pinched line, improper pressure control, too much sediment in the tank (starving the intake side of the pump).

I'm not familiar with a C4 fuel system but I am with some other makes. A problem that happens on other models is that the high speed side of the pump power supply goes bad. So.. at lower fuel demands everything is perfect. But at high loads (where more fuel is commanded) the engine starves because the power given to the pump doesn't increase. (on some fords its a simple resistor that gets overheated and burnt out)

antfarmer2

I would tape a fuel gauge to the windshield and run it might just need another fuel filter

Bugly87

Thanks for the replies. I will check the fuel pressure tomorrow after work. The pump and assembly are new so I'm hoping it isn't defective. Can a fuel filter clog after less than 3 years and less than 5000 miles?

Thanks again and I will report back with results

Kevin

1984Z51auto

Fuel pump could be fine and filter or sock blocked up with trash from one bad tank of gas. Also a hose could be soft inside the tank or partially blocked and coming apart from the inside. These things are all about 20 years or more old. In all of those instances, low demand fuel pressure could be good and a restriction could still drop pressure at high demand when accelerating or at WOT.
To answer your other question, pull the Throttle Body air intake hose and have someone floor the throttle (engine off!) and check that the blades open to horizontal. Doubt this is the issue but, strange things do happen.

BigDaddy5

Just a different direction to check, although I'd lean towards fuel pressure as well. My '85 had the exact same symptoms as yours, wouldn't get any faster than about 85-90mph, and felt like there was a governor. You did mention the EGR valve...issue on mine was the emissions, part of the catalytic converter had broken free, and partially blocked the converter. I'd lean towards the fuel pressure for now, since that's easier to troubleshoot, but it's something to keep in mind if the pressure checks out.

FASTAZU

How is the low end power? How is the exhaust flow? Yes it could be a plugged up cat. Also overdrive has no power Drive on an automatic is gear you should be in for what you are doing . Did your trans downshift back to Drive?

wydopnthrtl

It's unlikely but.. 1 tank of gas could plug up the fuel filter.

btw, I had a pontiac one time that had a plugged cat. It was my daughters car so I can't give many details.. but she was noticing it took a lot of throttle to on the hwy. New cat and it was fixed.

Bugly87

Thanks for the reply. My mechanic told me my EGR valve is bad. They plugged the hose and the hesitation went away. I ordered an AC Delco EGR valve from Rock Auto. Should be here Saturday. I also though plugged cat but don't know how to diagnose. The exhaust gas is even on both sides although I assumed the exhause should be more forceful. I will go in that direction after the EGR replacement and fuel delivery

Bugly87

Low end power seems fine. Expected pick up and power through the gears. Exhaust flow is even but I thought it should be more powerful. Yes the trasns did shift down to drive. On that topic how high is it safe to rum your rpm in drive on the highways? Any suggestions on diagnosing the cats? I installed a new 02 sensor last week with no improvement

Bugly87

Thanks for the info. New fuel filter last weekend. I'll check the cats this weekend. Any suggestions on hold too. The meinikee guy told me I need mufflers since the leak at the welds. He knocked on the cats but lead me to believe that the are ok. I know I need mufflers and a pipe just forward of the mufflers. Could this cause such a performance issue. I didn't think that would be an issue since they are cat back
Thanks for any replied

Kevin

wydopnthrtl

I'm a bit surprised the egr would cause that much power loss.

The best way to test for a plugged exhaust system is to tap into the pipes and measure back pressure.

FASTAZU

Your engine RPM's are limited by the rev limiter, the only thing you will hurt by running in drive is your MPG. Stand behind your car and have someone rev it the flow should be strong and fast, if its low and hot you may have a cat issue. I will say most cars with stopped up cats also had issues of low/lack of power when doing a hard acceleration from a stop.

Bugly87

I would classify my exhaust as low and defintley hot. I did do that test. I compared to a 4 cylinder Jeep which was stronger and faster. Thank you for the help

BigDaddy5

Easiest way to diagnose is to just remove the cat and look, lol! That's what I did, took about an hour after knocking the rust off the bolts. Even exhaust flow wouldn't really tell much because it's a single cat before the y-split, so it wouldn't really tell you if the cat was plugged.

Again, it would seem more likely to be the fuel delievery system...that was the first suspect when I had the same issues.

Bugly87

I checked my throttle body flap and it opened to horizontal at WOT. I'll be changing my EGR tomorrow and report back. If it ends up being fuel delivery is there a high performance pump that is recommended? Ive installed two pumps already with no improvement. Thanks for the help thus far

Kevin

1984Z51auto

OP, I don't see where the fuel pressure under load has been checked. You really need to make sure it is getting full pressure at WOT now that you are sure that the throttle is going fully open.

Bugly87

I had the opportunity today to check my fuel pressure on the highway.

In drive I hit 70 MPH at 2800 RPM's with 38 PSI...ran ok

In drive I hit 73 MPH at 3000 RPM's with 38 PSI...ran ok

In drive at WOT I hit 75 MPH at 3100 RPM's with 40 PSI...miss and stumble.

Slipped it into OD and hit 72 MPH at 2000 RPM's with 40 PSI...ran ok

Accelerated to WOT in OD...transmission kicked down hit 80 MPH at 3200 RPM's with 40 PSI...miss and stumble..knocking heard here

slowed down in OD to 75 MPH at 2100 RPM's with 38 PSI...ran ok

Accelerated to WOT in OD and hit 80 MPH at 3300 RPM's with 40 PSI..miss and stumble

slowed down in OD to 73 MPH at 2100 RPM's with 37 PSI..ran ok

At idle I maintained 36 PSI

Not knowing to much I would assume that my fuel delivery at higher speeds is ok. I ran out of time so I will replace my EGR valve as soon as possible and report back. Looks like I am closer to clogged CATS.

Any suggestions and thanks for all of the help,

Kevin