I first posted this under C4 Tech/Performance and within a matter of minutes over 20 people had viewed it. Unfortunately those 20+ people choose not to respond, so now I am posting here. I hope I get a better response.

What does a complete tune up on a '91 (L98) engine entail? I have previously owned two Corvettes but they were both pre '81 C3s and as you may know, a tune up on a C3 is a simple process. I am under the assumption that a tune up on a C4 is a much more complicated matter.

Also, what is a good indication that a L98 (C4) is in need of a tune up, would it be based on mileage or performance?

The car has about 4000 miles on an engine that was professionally rebuilt at about 43,000, it runs well most of the time but on occasion, when I get on it, it seems to bog out, as if it is getting to much fuel or not enough.

If there is any input that you can provide, it is greatly appreciated.


[Modified by Mr. Sharky, 9:27 AM 4/5/2002]

 

Distributor Cap-Rotor-Plugs-Spark Plug Wires(I like the MSD 8.5mm avail from Summit) Fuel Filter, K&N Air Filter and a new PCV Valve. That should get you in good shape. I just had the Hypertech coil installed and I like it a lot. Summit sells this as well for our cars.

Yeah, I don't care for that view option. Call me a control freak...but I can't stand to see no replies..but a million views. :rolleyes:

 

Yep JD covered all the pieces I would replace

 

Thanks JD 90 383

Based on your information, tuning a L98 is no different than a L82. I had assumed that it would be more technical because of the computer system.

What about oxygen sensor, TPS Voltage and injectors?

Could my bogging problem be caused by anything other than a need for a tune up?

 

The O2 sensor is an excellent part to replace for a tune-up that is commonly overlooked. You have a single O2 sensor on the driver's side y-pipe just after the exhaust manifold.

The TPS Voltage you can check with a multimeter and 2 needles, or you can buy a $14 harness adapter that plugs in to it so that you don't need to puncture the stock wires.

 

You seem to be on the right track. I have read where TPI cars are very sensitive to the system being clean. Any residue in injectors can hurt. I would change filter and use injector cleaner. Also the Batt can effect the voltage in the system. Since you're not having problems, change fuel filter, run a can of injector cleaner in tank.

 

Based on the information I have so-far received, I will start by changing the fuel filter, oxygen sensor, PCV valve, and air filter. I will then run a can of (STP) injector cleaner through the fuel system.

If this does not correct the problem, I will move forward with the more costly and time-consuming aspects of the tune up process.

 

Sorry forgot the 02 and pcv valve. I don't even know why my car has an 02! It runs in open loop and doesn't take 02 reading? or something like that.

glad to be of assistance.

 

You probably will want to check the timing also. Stock is 6 degrees BTDC. You must disconnect the brown EST wire before setting the timing so that the computer won't try to pull out any on you while you are adjusting. The distributor is a 9/16" bolt.

 

I’m confused, I just got of the phone with the Corvette shop, and the mechanic told me that I should not need a tune up after only 4000 miles. (I thought I needed one, for sure.)

He went on to say that my bogging problem should be tripping some sort of malfunction code and it is not. I don't understand, if I don't need a tune up and I am not getting a check engine code, what's the deal?

 

I wouldn't buy what He said. tune ups are definitely required. If it doesn't solve your problem, you at least know your ignition system is up to snuff. It's a good process of elimination starting point as well.

You might want to change your fuel filter. Mine was clogged to hell and I definitely notice easier starts,and smoother running. Keep us posted.

 

The General recommends NOT USING fuel injector cleaners. It seems that the coils in the injectors are cooled directly by the fuel and injector cleaners act as a solvent on the insulation of these wires causing the circuit to eventually short out ruining the injector! The injectors are supposed to be self cleaning.

 

Thanks, I didn’t know that. I would hate to harm my Corvette while in the process of trying to repair it.

 

After being in storage for the winter I took the Corvette out for a test romp, over the weekend, it performed fine. I am stumped as to why the bogging only happens periodically.

However, you are correct, a tune up would be a cheap and routine starting point. I plan on giving it a complete tune up and changing all of the fluids and filters, when I take it out of storage for the summer.

 

Did you replace the fuel filter? I had a problem with hot starts...and when it was stored outside in freezing temps, the idle would surge like mad. Since the filter was changed...hot starts are effortless and the surging is gone. You wouldn't believe what kind of crap gets lodged in there.

Change it,and let us know how it looked. That alone is more than likely going to improve things.


[Modified by JD 90 383, 6:18 PM 4/9/2002]

 

I agree with JD, the fuel filter is too often overlooked. also your throttle response is directly affected by the TPS voltage so that is an area that should be addressed first. :cheers:

 

The items mentioned so far are mostly good routine maintenance. Since you are having occasional issues you may want to do some diagnostics.

Your computer (ECM) has a diagnostic mode. Use a paper clip to jmper the ALDL (under the dash to the right of the steering wheel) pins A&B (upper right) and start the engine. After it warms up the SES light flashes will slow to about 1/sec. The On/Off periods indicate Rich/Lean condition. If the periods are of equal duration the ECM is in closed loop mode and the air/fuel ratio is correct. Then raise the RPM to say 1,500rpm and check again; repeat at 2,500rpm. If all is well the O2 sensor is healthy; don't replace it.

Turn the engine Off. Then turn the ignition On but don't start the engine, with the paper clip still installed. The SES will now flash "12" three times, followed by any trouble codes three times, and finish up with three more "12"s. If it just flashes three "12"s followed by three more "12" there are no lurking codes. BTW, do the timing check before this test as running the engine with with EST wire disconnected will set a code "42". To clear any trouble codes, open the connection in the Orange wire near the battery or pull the ECM fuse for about a minute.

Since you have a '90 L98 I don't believe that your TPS is adjustable. But you can pull the connector and use a DMM on the TPS terminals to measure resistance as you smoothly move the throttle from idle to full open position. The readings should increase smoothly with no flat or "jittery" spots.

 

Thanks for the information, I will try this procedure over the weekend, if I get the opportunity.