I didn’t want to be that guy that puts up another thread of something already up but here I am. I’ve not seen original posters looping back to say what they found that fixed the problem. 1988 C4 52,000 miles. This car has been in our family since early 1990. Me, my older brother and now my son who bought it in April 2019. Shortly after bringing it home to Arkansas from North Carolina we got a code 34. New MAF sensor fixed that. On a trip my son said it would not accelerate to pass. The car was just not responsive to the accelerator when passing. The problem seemed intermittent at first. Also got a Code 32. EGR temp switch seemed to fix that but still the "no power" problem when heated up. I’ve been using a Snap-On MT2500 scan tool I bought to troubleshoot this but it hasn’t revealed a real problem to me. (I‘m not a professional mechanic.) I also have the original 1988 Corvette FSM I bought when I got the car. No codes and all sensors seem to be doing what they are supposed to do. I have not taken a “movie” of the codes the Scan tool allows. I need to do that. For various reasons I’ve: Replaced the TPS and calibrated it, checked the fuel pressure in all situations including running and at full throttle. The fuel pressure holds up. Replaced the plug wires, distributor cap, rotor and timing module. Replaced the EGR temp switch and checked out the EGR Valve and EGR solenoid. Sent the Lucas injectors to Fuel Injector Connection for cleaning and servicing. I’ve checked the injector coil resistance with my FLUKE meter. 7 are 17.3 Ohms hot, 15.9 cold and one 15.3 hot and 14.9 cold. Hot readings taken right after engine shutdown. I’ve had the ECM checked out and the Prom re-flashed and checked. When I got the ECM and prom back it ran great. Full power at all throttle levels until the engine completely warms up after 30 or 40 minutes of driving. Then it becomes non responsive to the throttle. I’ve checked the grounds and cleaned them more than once. What can heat affect in these models systems to cause the low power at aggressive throttle settings yet no codes? I'm assuming it is heat because then is when we have the problem. Could the injectors still be bad after looking so good electrically? Sorry for the length of this. I’ve tried to condense all I’ve done. I may not completely understand the computer system in this model car I think I know what it does and doesn't do but maybe wrong.

3-10-2020 More words:
I felt like the catalytic converters were clogged so I replaced the whole exhaust system. I thought that based on inlet and outlet temperatures I took of the converters while the engine was running. That didn't fix the performance problem. Regardless, it needed a new exhaust system so I'm glad I did it. I took a "movie" with the Snap-On MT2500 when the low power occurs. Driving at 60MPH, press the throttle like passing a car, as soon as it downshifts it just has no acceleration or power. Let up on the throttle and the engine smooths out. When I watched the data in the scanner movie when I pressed the throttle I could see the transmission downshift but the injectors didn't respond with more time on. I watched the injector command go from 4.9 to 5 to 6ms as I pressed the gas. when I hit that downshift point the injector milliseconds went to 4.9 when it probably should have gone to 10 or 11ms. No fuel, No acceleration. I assume the millisecond times I am seeing on the Scan Tool is a command from the ECM to the injectors. Does anyone know if that is correct? I don't think it is from lack of fuel TO the injectors is it? I stated above the fuel pressure was good but I bought a new gauge today and checked it further. At idle I am at about 33 PSI. rev it up and it goes to 40 PSI. The pressure goes from 33 to about 40 while driving. I don't think it is supposed to be below 37. When I put my foot in it the pressure goes to 40 and stays there. The 40 number is OK I think. The lower number is not I don't believe. I still don't get the low injector millisecond firing reading. I cleaned and checked all the grounds again. Any ideas would be appreciated. I believe If I were a real Corvette mechanic instead of a shade tree one I would have this figured out by now. Thanks

4-24-2020 PROBLEM SOLVED: I wanted to report back on the low power issue with my 88 Corvette. It was a bad "Mass Burn Off Control Relay". I didn't find it. A young man in town that has a trouble shooting business found it. I knew the MAF relays existed but never suspected them as I got what I thought were reasonable MAF reading on my Scan Tool. This is where experience and the right equipment comes in. He found a low voltage that didn't make sense with his oscilloscope. The contact points on the relay were bad. There are two MAF relays. One is the MAF Burn off Relay and the other is a MAF Burn Off Control Relay. The control relay was bad. I have learned a lot about this engine on my journey to the solution. I'm glad for that but not for the parts my son had to buy while I was working on the car. The symptoms all resembled things that can happen from several components. It says in the GM Service Manual a thorough understanding of the engine and systems operation are needed to accurately trouble shoot engine problems. I don't know if I would have ever gone to these relays on my own. Sometimes you just have to call in the pros. Thanks to everyone that offered suggestions and advice.

 

Check you catalytic converter.

 

Thanks, James. I did that and it flows per the service manual.

EDIT: James, I checked it again and it's clogged. I'm replacing the exhaust system. Should have done that anyway. thank you.

 

You're going to have to take that snap shot of the data when it acts up. What fuel pressure reading are you getting when it acts up? Just curious. 30 to 40 minutes running, so this thing is more than hot enough. When it does act up, is it like a flat spot? Does it ping at all? Also, does the torque converter clutch disengage when you accelerate? You should see that on your scan tool..

 

How does the car idle once fully warmed up??

 

I have taken another look at my catalytic converter and it's clogged. I'm putting an new exhaust on the car. I'll report back. Thank you for commenting.

 

It idles fine and starts quickly cold or warm.

 

I checked fuel pressure today again. when I accelerated or attempted to the fuel pressure came up to 40. It is a flat spot. when pushing as if to pass a car, it feels like it downshifts and immediately just flattens out. I took a movie and the injector command dropped to about 4.6ms. when it pushed the pedal. this is down from 6ms. just before that. I'll look for the torque converter switch tomorrow on the snap shot. What would cause the injector command to drop when the throttle is advanced significantly? On the rate occasion when it does pull hard to the top I noticed the injectors were at 11ms. or more. The ECM tells the injector what to do, correct? The MT2500 reads that command, correct? Low fuel pressure wouldn't' affect the injector command I am seeing would it? I mean if the fuel pressure was low the signal to the injectors would not be less millisecond time. The engine isn't getting enough fuel from what I can see. The injectors are not on long enough I suppose. Thank you for responding.

 

What is your MAF sensor readings. That is a load sensor to tell the computer how much air is going in. If that is off, and they do go off from time to time it will cause that flat spot.

 

Here is what the Scan Tool reports. I started the "movie" about 4 frames before I accelerated or attempted to. at Scene 5 is when the Torque converter went out of lock and the performance flattened. I hope the formatting holds up as I try to list the readings. I've also attached a picture of the spreadsheet if it doesn't. The oxygen sensor is brand new. The MAF sensor was replaced about 9 months ago. The MAF readings and the air flow readings always match.
Scene RPM Speed MAF Inj. Ms. TPS TCC 4th Gear 02 Sensor Spk. Adv. Load Block Learn
1 1875 65 1.22 P1 P1 547 36 128 116
2 1925 65 1.80 P2 P1 27 36 137 116
3 1900 65 2.46 P2 P1 4 36 137 116
4 1825 66 52 6.0 2.84 P2 P2 4 35 134 116
5 1975 66 53 5.8 2.94 P1 P2 4 37 128 128
6 2225 66 52 5.2 3.36 P1 P2 4 40 115 128
7 2300 67 51 5.0 3.60 P1 P1 4 40 107 128
8 2225 67 51 5.1 3.96 P1 P1 4 41 111 128
9 2525 67 51 4.4 4.00 P1 P1 4 41 95 128
10 2475 66 50 4.6 3.94 P1 P1 4 41 96 128
11 2450 66 50 4.5 3.82 P1 P1 4 41 97 128
12 2475 66 50 4.6 3.75 P1 P1 4 41 102 128
13 2475 66 51 4.5 3.62 P1 P1 4 41 102 128



IMAGE OF SCAN TOOL DATA

 

If your reading of the O2 sensor is correct, a reading of 4 millivolts indicates a lean condition. Even though fuel pressure is OK, have you checked volume? When was the fuel filter replaced?
Also, is this thing going into closed loop? I see your block learn is midstream,

 

Yes it is in closed loop and yes It is lean during these scenes. I assumed that was because the injectors were not opening long enough. The fuel pressure is within the tolerance the FSM gives but it is on the low side of the tolerance. Maybe the pump is weak? I know the relationship between pressure and flow and have wondered about the pump. Maybe the sock in the tank is clogged and creating a restriction? I put an new fuel filter in it about 6 or 7 months ago when it started acting up. The old one was not clogged at all. I was going to pull out the fuel pump today but waited until I showed you the numbers thinking something else may reveal itself to you. If I undersand it correctly the ECM commands the fuel injectors and the millisecond drops and is low per the throttle position voltage. At this TPS voltage when it was running well one time the injector Milliseconds was around 11 or greater and the MAF was flowing well over 100. The engine was pulling and breathing well when it got the fuel it needs.
So, if the fuel pump isn't supplying the volume of fuel will the ECM's signal to the injectors be decreased. The low injector command by the ECM may be due to other sensor input it is getting that is telling it to do that. Hell, I've tried about everything else. I might as well put in a new fuel pump. Would that be a next good move? Thank you for your replies.

 

Might I suggest a measurement of Gasoline "Volume" flowing through the Fuel System. My 1988 C4 Fuel "supply" line Corroded Closed Internally allowing me to see correct pressure but In-sufficient Volume to allow the car to run at all speeds.

I disconnected the fuel line at the fuel filter. Then measured the fuel tank output from the pump. Next I removed the fuel line coming from the tank and attached a rubber hose from the "tank output" to the fuel filter and the car ran great. I don't remember the exact amount but it was close to double the volume that the car was able to get through the factory fuel supply line.

Today's gasoline with Ethanol really helps the steel lines corrode as it allows water to be absorbed and starts rusting in spots. Just remember that your brake fluid also absorbs water and those lines rust out without warning.

That O2 data is pretty conclusive it appears. Any time they are over 24 months old they need to be replaced. It is hard to see the signal as it oscillates between .1 and .9 volts very quickly when new or functional. Older units loose their signal strength and lag behind the Corvette's Computer.

Being an inexpensive component that is so "critical" to the Performance of a Corvette and helps you get better mileage I wonder why so many people drive around with worn out 02's. They might "still" work but here at my household my OBD1 cars get a new O2 every two years. OBD2 is designed for 48 months or 100k miles. The three I refer to are the Coolant Temp Sensor, The Oxygen Sensor and the Mass Air Flow sensor and they all play a very important role.

The MAF sensor's wire can sometimes "recover" with a good cleaning but be very careful as the wires in the MAF that measure the flow are very tiny in diameter and can easily be broken or damaged.

Best regards,
Chris

 

Thank you. There is a procedure in the FSM to check fuel volume. I'll do that tomorrow.

 

24 months is all an O2 sensor is good for?

 

No, an O2 sensor can last decades before it dies. My 86 ran on the original sensor for 30 years. My 88 is still on the original sensor.

It doesn't hurt to replace it if its failing but I'm not convinced from the data posted that it is. I'm leaning more towards fuel pump or another delivery problem.

 

Hi Darth, the oxygen sensor is brand new. I installed it two days ago when I put a new exhaust system in. These readings were with the new sensor. Agree with fuel pump or delivery issue. I'll work on that tomorrow when I go back over to my son's shop where the car is. The old sensor never read bad. It delivered readings all up and down the scale. I replaced it anyway. They are not expensive.

 

At any time does your block learn go above 128? That's actually the mid point of fuel management. When you accelerate, MAF and fuel inj pulse should increase together, as that happens, the block learn should go above 128, which means the ECM is trying to add fuel. Based on your data, and your description, if the ECM doesn't command above 128, it can't add fuel. I would hold off anything in the fuel dept, until we know the ECM can command more fuel. If you can't get it above 128, there might be something going on with that prom. But take it on a road test to see if, at any time it goes above 128.. If not, I have a few things to do that will trick the ECM to see if it can adjust.

 

OK, Mrvettenick, I'll do that. I sent the Prom to a fellow that can read them in December. He said some of the bin files were corrupted or missing. Said he re-flashed the PROM. I didn't know you could do that to a Prom chip. That is rewrite on them. Anyway, when I got it and the ECM back from him the car ran great for about 2 days. Now it's back to the original Issue that started all of this about 6 or 8 months ago. I'll check the Block Learn and open or closed loop question you asked me. I read a book I have on Corvette fuel injection that at 80% to WOT the ECM goes back to open loop and doesn't pay attention to the 02 sensor. Just gives it fuel and says "to heck with fuel economy, give us power". (Corvette Fuel Injection 1982 thru 2001 by Charles Probst, page 32) I understand PROM's seldom go bad. That's what I read. For what ever reason the ECM isn't telling the injectors to deliver more fuel via longer pulses. Don't know how that can be a result of low fuel pressure or volume but I'll try about anything at this point. (I know enough to be dangerous. Think that's obvious)

 

Thanks for your response! IMHO, fuel pressure is not my primary concern, especially if the ECM can't extend injector pulse width.