Topic: 1990 L98 TPI runs rich
in Forum: C4 Engines
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Is there any way to adjust the mixture on the TPI? It has always run rich, can smell bass
In the exhaust.
Over the years I've replaced the injectors, control brain, and tried an adjustable fuel pressure regulator. None has fixed the problem.
Any advice would be appreciated.
Geo
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Hi,
no there isnt and ther cant be. the fuel mixture is adjusted by the ECU taking into account a lot of things, esp engine temp and the readout from the oxygen sensor.
if the L98 runs rich, you probably should have a error code (DTC) stored in the ECU. did you try to access the error codes?
this would be essential as just buying parts is a very expensive way to try to troubleshoot a fairly modern computerized engine. what you may try to do... disconnect the ox sensor with engine at operating temp. this should cause the ECU to revert to "emergency" settings, which will be somewhat alright, but no longer controlled. same with the HFM air flow meter.
you may want to check the resistance of the eng temp sensors (coolant and air), the data are in the workshop manual.
remember... with this kind of engine, there is no valid reason for strange behavior, any deviation from the factory settings will be due to a problem.
good luck!
Wolfram
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Huron, OH - USA
Joined: 12/5/2009
Posts: 179
Vette(s): 1989 Red Convertible - 160,000 fun-filled miles -
My 89 Vette had the same "rich smell" for years. Even after replacing the fuel injectors and ECM when they failed, and a host of other normal maintenance and tune up items
needed to keep a 150,000 mile card going. Then a few years ago I replaced the entire exhaust system, all pipes, mufflers, CATS, and the oxygen sensor. After that project, I've never had the gas/rich running smell show up again. Can't say which of the exhaust components was causing the rich smell, but it was one, or a combination of them (still can't believe I got 25 years and 150,000 miles out of an exhaust system)
I recently had an issue on my '94 LT1 running rough. The exhaust smelled rich but the code from the scan tool indicated "Lean Condition on Right Bank."
Well, digging deeper, using the code reader to look at the engine parameters while the engine was running I saw very low millivolt readings on the right O2 sensor. It read 90 millivolts and didn't change much at all. The left O2 sensor was reading from 900 millivolts to around 100 millivolts. That is what it should be doing. It is measuring excess oxygen in the exhaust and the voltage it puts out goes back to the computer to allow the computer to adjust the fuel/air mixture by regulating the duration of the fuel injector pulse. (the longer the pulse the more fuel delivered)
A low voltage from the O2 sensor would tell the computer the mixture is lean because there is excess oxygen in the exhaust. The computer then increases the fuel by increasing the duration of the pulse to the fuel injectors. When the mixture is enriched to the point that there is more fuel than can combine with the air there will be no excess oxygen in the exhaust and the O2 voltage will go up. That in turn signals the computer to reduce the fuel delivered. The computer reads the voltage from the O2 sensors many times a second and adjusts the fuel delivered. This is the closed loop mode of operatoin.
I noticed that when the Check Engine Lamp would come on the car would run better. After a while of having the Check Engine Lamp on it would go out and the car would go back to running rough. I finally figured out when when the computer got the lean code the computer goes into open loop mode where it uses fixed settings for the fuel mixture. For instance, when you start the car cold, before the O2 sensors heat up and start giving good readings, the computer runs the engine open loop with preset mixture and timing settings. Once warmed up the computer "closes the loop" and starts regulating the mixture based on the O2 sensors measurement of excess oxygen in the exhaust stream.
In my case the low voltage from the failed O2 sensor told the computer the mixture was lean, so the computer increased the fuel delivered. But since the O2 sensor had failed it continued to indicate a lean mixture. At some point the computer decided that it couldn't deliver enough fuel to get a proper mixture on the right bank and turned on the Check Engine Lamp, dropped a code into the memory, and went into open loop mode.
So, you might want to take a look at the O2 sensor readings with a scan tool and see what they are.
Bob
Woodstock, GA
'94 LT1 Polo Green
Well, digging deeper, using the code reader to look at the engine parameters while the engine was running I saw very low millivolt readings on the right O2 sensor. It read 90 millivolts and didn't change much at all. The left O2 sensor was reading from 900 millivolts to around 100 millivolts. That is what it should be doing. It is measuring excess oxygen in the exhaust and the voltage it puts out goes back to the computer to allow the computer to adjust the fuel/air mixture by regulating the duration of the fuel injector pulse. (the longer the pulse the more fuel delivered)
A low voltage from the O2 sensor would tell the computer the mixture is lean because there is excess oxygen in the exhaust. The computer then increases the fuel by increasing the duration of the pulse to the fuel injectors. When the mixture is enriched to the point that there is more fuel than can combine with the air there will be no excess oxygen in the exhaust and the O2 voltage will go up. That in turn signals the computer to reduce the fuel delivered. The computer reads the voltage from the O2 sensors many times a second and adjusts the fuel delivered. This is the closed loop mode of operatoin.
I noticed that when the Check Engine Lamp would come on the car would run better. After a while of having the Check Engine Lamp on it would go out and the car would go back to running rough. I finally figured out when when the computer got the lean code the computer goes into open loop mode where it uses fixed settings for the fuel mixture. For instance, when you start the car cold, before the O2 sensors heat up and start giving good readings, the computer runs the engine open loop with preset mixture and timing settings. Once warmed up the computer "closes the loop" and starts regulating the mixture based on the O2 sensors measurement of excess oxygen in the exhaust stream.
In my case the low voltage from the failed O2 sensor told the computer the mixture was lean, so the computer increased the fuel delivered. But since the O2 sensor had failed it continued to indicate a lean mixture. At some point the computer decided that it couldn't deliver enough fuel to get a proper mixture on the right bank and turned on the Check Engine Lamp, dropped a code into the memory, and went into open loop mode.
So, you might want to take a look at the O2 sensor readings with a scan tool and see what they are.
Bob
Woodstock, GA
'94 LT1 Polo Green
Bob - '94 Polo Green Coupe
Most O2 sensors ideally should read 450ma when fuel/air mixture is correct. 800ma would show a rich condition.
I have a Polo Green 94 as well and it runs better when the O2 reads a little higher( 550-600ma).
Right now I have other problems, my Optispark is malfunctioning so I'm going to tackle that now. Hate the way they buried the thing where it is but just being able to drive that baby makes it well worth it.
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Equinunk, PA - USA
Joined: 11/12/2017
Posts: 76
Vette(s): 1993 LT1 convertible triple black 4L60
2003 Zo6 Electron/Black 6 sp
Are you pulling the engine, or the rack to access the Opti unit. They are a BEAR to replace.
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Sykesville, MD - USA
Joined: 3/20/2009
Posts: 8
Vette(s): '86L Coupe
Factory ordered 1/86
Sold 11/88
Found 12/07!
2016 Z51 Coupe
Night Race Blue
My'86 ran rich but threw no codes.
I finally found that my gas tank was building pressure after a nice long drive in warm weather.
And I'd get a whiff of gas sometimes while driving. I tested and replaced the purge valve and all hoses to the charcoal canister and no more problems! My autoXray shows normal fuel mixture too.
You don't have to pull the engine or the steering rack to get to the distributor. You do have to pull the water pump and the crank shaft belt drive pulley, but both can be done without removing the rack or engine. Been there, done that. I had a dribbling water pump and got to it before is screwed up the OptiSpark. I was able to get by with just changing the Cap and Rotor on the OptiSpark.
If you are going to go that far, go ahead and put a new water pump on while you are there. If the pump starts leaking you are likely to kill the OptiSpark unit.
If you are going to go that far, go ahead and put a new water pump on while you are there. If the pump starts leaking you are likely to kill the OptiSpark unit.
Bob - '94 Polo Green Coupe
in Forum: C4 Engines
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