Hey all, I'm new to the world of vettes and also new here on this forum. so a couple of days ago my brother brought me a stock '89 vette w/ a 6-speed manual to work on that seems to have some rather interesting acceleration issues. we drove around in it for a few minutes so he could show me the problem. whenever he would accelerate from a low rpm the engine would accelerate until ~2200rpm, and from then on it would stumble and finally reach at most ~3250rpm. if he punched the gas to the floor would not accelerate at all and it would act like it was at idle. when we pulled into a parking lot he revved the engine and it smoothly went up to near redline without missing at all.

First we did a general tune up that consisted of replacing the sparkplugs, wires, distributor cap and rotor and fuel pump, and then set the timing. The car idled smoother but it stumbled just the same as before. Next I replaced the injectors with accel 30lb/hr injectors. after installing those the car idled even better but the stumbling is still there. after a bit of reading i checked the error codes and got a code 22,33 and 42. I'm guessing the 42 was from setting the timing and the 22 and 33 from unplugging the tps and maf to see if either of those made a difference(neither did make a difference in the end). after resetting the ecu and running the car in park until it went into closed loop mode, the car still exibits the same hesitation and lack of power.

at this point i have no idea what to try next. any suggestions? i would love to get this car back to my brother in a drivable state, and any input would be greatly appriciated.
thanks,

christian G

 

Have you performed any diagnostic tests, or simply throw parts at it?
First thing I would do (before buying a boat load of parts) would be to check:

Voltage
TPS setting and range
Timing
Fuel pressure with a gauge taped to the window.
Vacuum with gauge taped to the window.
If you have not done the above, you may want to now.

30 pound injectors without reprogramming the prom is a step backwards. Factory was 22 pounds or so.

 

hey, thanks for the info and good advice.

the voltage is good, and the timing was already done. 3 of the stock injectors had low ohms so we just replaced them all anyways, though i do agree that stock replacements should have been used, but alas i didn't buy the parts. as far as the TPS goes, is it a standard resistance value type, as in a 5volt in, a ground reference and a varying output? forget it, I'll use the search feature. do i check the fuel pressure at the schrader valve?

thanks in advance for any more advice.

 

The schrader valve is where you want to screw in the gauge.
With 30# injectors, every time they fire, they are shooting a ton more fuel with the stock tune.
TPS will start at about .5 volts and sweep to approx 4.5 volts at WOT.

 

The TPS voltage starts at .51 volts and climbs smoothly to 4.26 volts @WOT, which is in line with what you said above, so i don't think that is the culprit. tomorrow i'll have to check the vacuum and fuel pressure. i didn't hear any surges when i sprayed some carb cleaner around the intake gaskets and vacuum lines, but i suppose that only means there isn't a large leak at idle. some of my brother's friends own their own shops so he might be able to borrow a fuel pressure test kit, and if need be reflash the prom and possibly dyno the car. Anyways thanks for all the input today, I really appreciate it.

 

Reset the ecu? I'm guessing that means you removed the neg. battery cable to clear codes. Have you pulled codes again? Would be interesting to see which codes,if any, returned.

 

At night and with the underhood lights off, engine idling, look along each plug wire for sparks. Replace broken or missing plug wire looms and don't allow plug wires to touch metal. Measure the resistance of each plug wire, if stock, they should be about 5000 ohms per foot.

 

yep, i pulled the neg battery cable to rewset the ecu, and right now all i get are the standard repeating error(not really error, but marker) 12 codes over and over. the wires are brand new accel's that i assembled and lightly greased with dielectric grease on all contacts, the resistance is good and there seem to be no visible arcs as i look at it tonight. tomorrow i want to hook up an led+resistor to both injector banks and see if the ecu just stops firing the injectors all together or not, along with a vacuum and fuel pressure gauge taped to the window. if everything looks good it has to be ignition, Right? do these cars have any history of bad HEI modules or other ignition issues i should be aware of?

 

The spark module inside the distributor will put out weak spark when they get up to normal operating temperature when they get old. They aren't expensive.

 

Do you have access to a scanner or laptop with Datamaster or similar ?

 

I might be able to borrow a scanner or build an ALDL serial interface in the next few days if I can find the time to do so. I'll have to call some friends of mine and scan it a couple hours from now. i hope one of the local parts stores has an HEI load tester so i can have the HEI checked before i throw any more money or parts at the car.

 

I had the same problem. Replaced the fuel filter and fuel pump and problem was completely solved. Replacing the fuel pump is super easy. Takes 20-30 minutes tops, comes out with the fuel sending unit. Replacing the fuel filter is a bit tougher. Mechanic friend told me that when I first replaced the fuel pump but not the filter, it was like trying to suck a cherry through a straw. Burns the pump up in short time.

 

Have you checked for plugged cat(s) ?

 

luckly i already replaced the fuel filter a few days ago so i don't have to go through that drama again, but i think you are right on the money with suspecting the fuel pump. at idle and with the vacuum line connected to the fuel pressure regulator i am seeing ~30 psi. at WOT in 2nd gear the pressure drops to ~7psi. If i disconnect the vacuum line from the fuel pressure regulator and plug it, at idle the pressure is around 50psi and @wot in second the pressure reads~10 psi. do you guys know what the actual specs should be? also the cats are new from the previous owner.

 

FP should be stable at 35-38 PSI.
During acceleration the pressure should increase to 40-41 then drop back to 35-38.

 

thanks JrRifleCoach, that definitely confirms my suspicions of a bad fuel pump. when the ECM goes into enrichment the fuel pump just can't keep up and the car stumbles. I'll have to replace the pump and see if that makes a difference. should i try to clean out the fuel lines while i am replacing the pump, and if so, do you have any suggestions on how to do so safely?

 

Replace pump, sock, and filter. That should do it. Check out the Racetronix kit.

 

 

I went through 2 fuel pumps and 2 filters before finally stabilizing my fuel pressure. I unfortunately now have an extended cranking issue that I believe is fuel rail related. Im guessing a leaky injector, which empties the fuel rail once I shutoff the car. My fuel pressure drops pretty quick after I shutdown.

Good luck on figuring out your issues...

 

Get a fuel pressure gauge to connect to the Schrader Valve on top of the passenger side fuel rail and tape the gauge to the windshield to watch it while driving.