Hi All,

New to C4’s. I have a 1990 Corvette. 53k miles always cared for and well maintained.

Took it out this morning. Not a mile down the road and I noticed the car sputtered and had power loss over 2k RPM’s. Brought it back, popped the hood all looked ok. Turned the car back on and with my hand engaged the throttle. As I’m turning it the sputtering came back and the car wouldn’t rev. I’m thinking there was a sensor or something silly that went bad.

Fuel pump appears to be humming in the tank. Car will always start. No check engine lights or codes on the dash.

Thanks for your help.

Best,
Dave

 

The miss and unable to freely rev I had just happen a bit over a week ago turned out to be the CTS. You can disconnect the CTS to see if that helps. Since you are not setting any codes that point to the CTS or any other sensor I don't have enough experience to help you in other directions.

 

You should monitor fuel pressure when it acts up. It should slightly increase when you move the throttle. Another possibility is the MAF sensor either dirty or acting up.
You'll need a scan tool to monitor it's activity.

 

Another possibility would be your catalytic converter has become "Partially Plugged". That would do exactly what you describe. To find out for sure you can pull the Oxygen sensor off the exhaust and see if it has a Huge volume of air trying to get out or just a little bit. It would probably help to get yourself the parts and to do a basic tune-up on the engine.

If your Engine runs fine in "Open Loop" and acts up only in "Closed Loop" that narrows down the possibilities. If it does it in closed loop then it would probably have something to do with one of the three primary sensors for closed loop operation.

One is the Oxygen sensor as it helps keep the air/fuel ratio accurate. If you don't know how old this sensor is the replace it. Oxygen sensors back in 1990 had a 24 month 50k miles lifespan. They might work afterwards but not as accurately. A lot of Oxygen sensors tend to fail "rich" to avoid damaging the engine. A rich running engine will damage the catalytic converter by plugging it up. When in doubt replace the Oxygen sensor as it is so critical yet wears out easily and is fairly cheap to do on an older OBD1 Corvette. A new O2 will bring back the performance and the economy of a Corvette. O2's were to be replaced regularly.

Two is the MAF or MAP. I have a 1988 C4 with the MAF. If you have one of these it is possible to clean the wire element with the proper cleaners. The MAF/MAP sensor are there to determine the volume of air being ingested into the combustion cycle of the Engine. These will drop out and affect the drive-ability of a car. On my L98 there are two relays that tie into the MAF system. One is for the "Burn-Off" function to clean the element wire of any build ups on the wire by heating the wire to a very high temperature. The second relay is there to "Power" the MAF sensor and is just as important. I recommend that folks replace the relays when the MAF is replaced as they are critical to the functioning of the MAF.
If a MAF sensor is to work properly you need to ensure that the hose that connects the MAF to the throttle body is sealed and that no extra air can leak in between the MAF and Throttle body, if this happens it will cause problems for drive-ability and the engine would not run properly.

Third critical sensor would be the Coolant Temperature Sensor which is what the ECM uses to determine the engine's operating temperature. If the CTS Fails and thinks it is Colder than it really is then the car would start easily when cold but not when Hot. Since the extra fuel is being injected (acting like a choke) the engine will flood easily when hot. The Hotter the engine is the less fuel is injected. If your CTS fails and thinks it is Hotter than it really is then your Corvette would be hard to start in cooler weather. The Coolant temperature Sensor is what changes the air/fuel mixture based on what it sees.

How old is the fuel in the tank of your Corvette? Today's ethanol/gasoline only last's about a month in your tank before getting nasty. I have found that "Mechanic In a Bottle" sold at Home Depot will awaken old fuels.

There are a couple of things you will want to get for this Corvette of yours. First buy a set of the Factory Service Manuals. Then buy a Fuel Pressure test gauge so you can verify that your fuel pressure is okay. A Scanner or OBD1 Code readers might be helpful. I have an old Snap On and it is wonderful.

Clean every connection that even looks suspicious. Always check your battery connections for good secure connections with no corrosion. I use a material called DeOxIt and it wipes away the corrosion leaving a clean connection, it is not cheap but it sure works!!

 

Thanks, will try.

Best,
Dave

 

Thanks, will put a scanner on it.

 

Thanks very much, will look into these. In my research a '90 has a MAP not a MAF. Is that correct?

Car had a recent tune up and driven regularly so bad fuel shouldn't be in the issue. Hopefully it's one of these sensors and everything will go back to how it was. On the code reader, if the o2 or CTS failed wouldn't it illuminate the check engine light? Or will it fail without notice?

Lastly, as I think back to this morning I think the Security dash board light was flashing and then went away. Would something security related influence this issue?

Thanks,
Dave

 

The security light blinking is normal, and go out after starting car.

 

On a scanner you would see the oxygen sensor not working or sending erroneous data. The oxygen sensor can fail without triggering a CEL. The CTS is something that you would have to recognize when you saw it out of range on the Scanner. I am not sure whether a bad CTS would trigger a CEL or not.

Oxygen sensors oscillate back and forth from .1 to .9 Vdc very, very quickly. They are hard to test without a oscilloscope or high end meter. As they age the signal gets smaller (Narrower) like .3 to .7 instead of .1 to .9 and then the signal starts to lag and this is when the drive-ability issues start since the O2 cannot keep up with the needs of the ECM. This is why I suggest that people replace them if you don't know how old they are. There are new ones that come out of the box dead as a door-nail.

Have you tried to see what happens when the O2 is removed from the bung on the exhaust pipe? I would try this one just to rule out a partially plugged catalytic converter. Don't let anything get on the tip of the O2 when it is out of the exhaust pipe as they are easily damaged.

 

Hi All,

Finally had a chance to spend some time working on the car tonight.

1 - Checked the codes. I had a couple H codes for the interior. Went though the computer and reset the codes. Then got C12 so all good as far as I know.

2 - Fuel pressure was in the high 30's with the key on. Started it up and rev'ed. Fuel pressure was below 45psi. I don't recall exact PSI but within the range as far as my research shows. What should the PSI range be? Does it vary at RPM?

3 - I dropped the cat and gutted it. No doubt it was badly clogged. Car sounds much better and idles better but unfortunately after a very quick road test the issue persisted. It's one bad miss. This time I could drive the car to about 2,500 - 3k RPM's before the issue happened. No backfires, no shutting off. When I got back to the driveway I put it in park and rev'ed again. This time it pulled to 4500 RPM. I'm at a loss.

4 - I have CTS, O2 sensor and MAP in hand. The plan tomorrow morning is to warm the car up. Check fuel pressure again more detailed and at various RPM levels. If all is good there I will replace the three sensors above. If the fuel pressure is not good then I will replace the pump and filter and retest. I also have a new coil but not sure I want to get into the ignition system. I really thought it would have been in the cat. The fuel pump, filter, injectors were all redone in 2014 (about 5k miles ago if I recall).

What else could it be and where else could I look? Any suggestions?

Thanks,
Dave

 

Don’t forget the easy things, a partiality clogged fuel filter can cause a normal idle and low speed performance with a loss of performance at higher RPM.

 

Once upon a time I let my C4 sit for about 18 months and the most unusual thing happened. The car was able to maintain fuel pressure at idle but was getting an insufficient amount of fuel to run any faster. It turns out that the ethanol in out new oxygenated gasoline will cause corrosion on the inside of the fuel "supply" and "return" pipes. They don't block off completely at first but over time they can and will.

When I finally started thinking along those lines I got a graduated container and measure the flow through the factory supply line and then I ran an external line and right away the volume was different even to the naked eye. It turned out that the car would run and be fine with a Fuel hose taking the fuel to the Input side of the fuel filter. The car runs fine now but I ended up replacing the fuel supply line.

This is getting pretty far out there on possibilities but with your situation it seems worthy of mentioning. You might try and see the difference in fuel volume going around your factory fuel supply line. I did it on my C4 by removing the tank filler cover and accessing where the hose connected to the fuel outlet. Take the other end and connect it to your fuel filter and see what happens.

What your car is doing sounds just like a lagging or defective oxygen sensor. Can you try another O2 before going to far?

The post mentioning a partially plugged fuel filter is a valid point, it could be a problem.

Keep us posted on the results of the tests you perform. We will figure this thing out yet!!

 

I would start with a basic tuneup. Plugs, wires, cap, rotor.

 

Thanks all. I was able to fix the problem today.

All in all each step progressed towards the fix. Gutted catalytic converter, replaced the CTS, replaced the fuel filter (and blew out the line). Car runs great and pulls hard.

Thanks again,
Dave

 

Good to hear you worked through the problem.

FYI, when you rev the engine FP should jump to around 45psig, with a clogged filter or weak pump FP will drop off as you increase RPM (went through this issue with my prior '88)

 

Thanks. What should the psi be when the key is on but not started?