nOOb question
Thread Starter
Melting Slicks
Joined: Nov 2021
Posts: 3,495
Likes: 16
From: Conroe Texas
2024 C4 of the Year Finalist - Unmodified
nOOb question
I recently picked up a new to me 1988 L98 which is my first Corvette, I am having an issue where when at idle at a light or stop sign and the car in drive the RPM's will dip from around 900 down to around 500 for just a moment and then back up. It doesn't do it over and over but it will do it a couple times while idling plus it has died on me twice because the RPM's dip too low while backing up, it runs great while driving with no issues and plenty of power. One thought I have is it looks like the previous owner either added and aftermarket air intake or he butchered it to allow more airflow causing the air to fuel ratio not being correct? he also added a cat back exhaust but not sure if that would affect the tuning or not since it is cat back but I assume it could? any help appreciated and I apologize in advance if this question has been asked upteen times 
Race Director
Joined: Apr 2010
Posts: 16,253
Likes: 1,657
From: Arizona
Welcome to the forum. If you know there is an air filter and other mods, perhaps the previous owner also added a Mass Air Flow sensor?
The one thing that seems consistent is, it happens as you are coming to a stop which validates a change in overall airflow.
Since this happens, can you test this out and come to a slower stop verse maybe a sudden stop? See if the problem persists if you slow down even slower. At least this may help you determine it is in fact more of an air intake issue depending how sudden you slow down.
You may be in need of going back to stock if it bugs you that much or possibly consider a tune?
The one thing that seems consistent is, it happens as you are coming to a stop which validates a change in overall airflow.
Since this happens, can you test this out and come to a slower stop verse maybe a sudden stop? See if the problem persists if you slow down even slower. At least this may help you determine it is in fact more of an air intake issue depending how sudden you slow down.
You may be in need of going back to stock if it bugs you that much or possibly consider a tune?
Thread Starter
Melting Slicks
Joined: Nov 2021
Posts: 3,495
Likes: 16
From: Conroe Texas
2024 C4 of the Year Finalist - Unmodified
Welcome to the forum. If you know there is an air filter and other mods, perhaps the previous owner also added a Mass Air Flow sensor?
The one thing that seems consistent is, it happens as you are coming to a stop which validates a change in overall airflow.
Since this happens, can you test this out and come to a slower stop verse maybe a sudden stop? See if the problem persists if you slow down even slower. At least this may help you determine it is in fact more of an air intake issue depending how sudden you slow down.
You may be in need of going back to stock if it bugs you that much or possibly consider a tune?
The one thing that seems consistent is, it happens as you are coming to a stop which validates a change in overall airflow.
Since this happens, can you test this out and come to a slower stop verse maybe a sudden stop? See if the problem persists if you slow down even slower. At least this may help you determine it is in fact more of an air intake issue depending how sudden you slow down.
You may be in need of going back to stock if it bugs you that much or possibly consider a tune?
Safety Car
Joined: May 2001
Posts: 4,720
Likes: 1,625
From: Fairfax Virginia
The 1988 C4 came with a M*** Air Flow sensor and they have a bad habit of getting dirty and sending wrong/incorrect information to the Engine Control Module. On the 1988 C4 the MAF has two Relays associated with it. One controls the "Power" and the second controls the "Burn-Off" Function. Getting the "Burn-Off" to happen is critical to keeping your MAF working properly. They use two tiny (1/10th of a mm) wires in the MAF that read the Volume of air entering into the engine's Combustion Chambers. The "Burn-Off" occurs when the MAF sensor heats the wires up to a bright yellow glow to burn off any contaminants in the process and this is controlled by the Burn-Off Relay. The second item is to be sure that tube that connects the MAF to the Throttle Body is sealed and there can be NO air leaking in anywhere. Any cracks or holes in the tubing makes it useless. Just be sure the tube is sealed and secure at both ends with clamps.
It sounds like you really need to start at the beginning and do a good Solid TUNE-UP on Your C4 just like suggested above.
First, do you have a Copy of the Factory Service Manuals? RockAuto has them on DVD for ~$35 and hard Copies can be had for about $85. This is a book set that you will NEED so get one if you don't have one yet. To tune up your L98 engine is more involved than replacing Plugs and wires like on older cars. The Throttle Body is where a lot of important things happen. There is Throttle Position Switch that tells the ECM the desired speed and it wears out. I would replace it with a good quality part and keep the old one. Then you have the Idle Air Control Valve which is a "controlled air leak" but can need attention. The IAC is what controls the idle speed in situations like yours, I would certainly look there before too long. When you buy a New IAC save the old one but be absolutely sure it has the same Pintle shape on both of them. NAPA tried selling me one with a newer type tip than my car came with. It seems that somewhere during the production year GM changed the Pintle Design to a "better" design.
I have a 1988 and have over 25 years with the beast. Have you got a Fuel Pressure Gauge on a long hose? They attach to the Schrader valve (on the passengers side fuel pressure line) and allow you to see the pressure changes while driving which can be helpful. How about any kind of OBD1 Scanner or even Code Reader? They are very handy and are able to save you a lot of time potentially. You want to be sure the Fuel pressure is within the recommended numbers, It is important to know that your Fuel Pressure Regulator is working properly. Find the device and remove it's vacuum hose and smell inside the hose, if you smell raw gasoline then there is a bad diaphragm in the FPR.
This engine operates in two basic modes. Open Loop is used when you first start the engine and it stays in Open loop until it either times out or starts getting a signal from the Oxygen sensor that should be warming up to operating temperature. Once the Oxygen Sensor starts sending it's oscillating signal the MAF and Coolant Temperature Sensor and O2 do "most" of the air/fuel calculations and this is called "Closed Loop" and in this mode the engine runs much cleaner and more efficiently as it is monitoring the O2 and adjusting the fuel accordingly. There are times that the car reverts back to Open Loop like wide open throttle for example, there are others. Even at long stop lights the older UN-Heated Oxygen sensors can cool down enough to force the engine back into "Open Loop". Then at a green light the L98 powered Corvette takes off and the O2 gets HOT fast and the engine switches back to Closed Loop again.
The Oxygen Sensor on your C4 was designed for a 24 month/50k usage and then it was replaced. They can and do work sometimes past their useful life but the signal gets narrower and slower over time. The New O2 will make your engine run more efficiently and you should have better performance depending on how old the original is. Besides they are cheap and easy to install.
Well Bottlejunkie you came to the right place for answers on how to "help" your Corvette! You are most Welcome to the Corvette Forum and we all look forward to hearing how your Corvette turns out!
It sounds like you really need to start at the beginning and do a good Solid TUNE-UP on Your C4 just like suggested above.
First, do you have a Copy of the Factory Service Manuals? RockAuto has them on DVD for ~$35 and hard Copies can be had for about $85. This is a book set that you will NEED so get one if you don't have one yet. To tune up your L98 engine is more involved than replacing Plugs and wires like on older cars. The Throttle Body is where a lot of important things happen. There is Throttle Position Switch that tells the ECM the desired speed and it wears out. I would replace it with a good quality part and keep the old one. Then you have the Idle Air Control Valve which is a "controlled air leak" but can need attention. The IAC is what controls the idle speed in situations like yours, I would certainly look there before too long. When you buy a New IAC save the old one but be absolutely sure it has the same Pintle shape on both of them. NAPA tried selling me one with a newer type tip than my car came with. It seems that somewhere during the production year GM changed the Pintle Design to a "better" design.
I have a 1988 and have over 25 years with the beast. Have you got a Fuel Pressure Gauge on a long hose? They attach to the Schrader valve (on the passengers side fuel pressure line) and allow you to see the pressure changes while driving which can be helpful. How about any kind of OBD1 Scanner or even Code Reader? They are very handy and are able to save you a lot of time potentially. You want to be sure the Fuel pressure is within the recommended numbers, It is important to know that your Fuel Pressure Regulator is working properly. Find the device and remove it's vacuum hose and smell inside the hose, if you smell raw gasoline then there is a bad diaphragm in the FPR.
This engine operates in two basic modes. Open Loop is used when you first start the engine and it stays in Open loop until it either times out or starts getting a signal from the Oxygen sensor that should be warming up to operating temperature. Once the Oxygen Sensor starts sending it's oscillating signal the MAF and Coolant Temperature Sensor and O2 do "most" of the air/fuel calculations and this is called "Closed Loop" and in this mode the engine runs much cleaner and more efficiently as it is monitoring the O2 and adjusting the fuel accordingly. There are times that the car reverts back to Open Loop like wide open throttle for example, there are others. Even at long stop lights the older UN-Heated Oxygen sensors can cool down enough to force the engine back into "Open Loop". Then at a green light the L98 powered Corvette takes off and the O2 gets HOT fast and the engine switches back to Closed Loop again.
The Oxygen Sensor on your C4 was designed for a 24 month/50k usage and then it was replaced. They can and do work sometimes past their useful life but the signal gets narrower and slower over time. The New O2 will make your engine run more efficiently and you should have better performance depending on how old the original is. Besides they are cheap and easy to install.
Well Bottlejunkie you came to the right place for answers on how to "help" your Corvette! You are most Welcome to the Corvette Forum and we all look forward to hearing how your Corvette turns out!
Advanced
Joined: May 2019
Posts: 94
Likes: 59
From: Harrison TN
Great Information
The 1988 C4 came with a M*** Air Flow sensor and they have a bad habit of getting dirty and sending wrong/incorrect information to the Engine Control Module. On the 1988 C4 the MAF has two Relays associated with it. One controls the "Power" and the second controls the "Burn-Off" Function. Getting the "Burn-Off" to happen is critical to keeping your MAF working properly. They use two tiny (1/10th of a mm) wires in the MAF that read the Volume of air entering into the engine's Combustion Chambers. The "Burn-Off" occurs when the MAF sensor heats the wires up to a bright yellow glow to burn off any contaminants in the process and this is controlled by the Burn-Off Relay. The second item is to be sure that tube that connects the MAF to the Throttle Body is sealed and there can be NO air leaking in anywhere. Any cracks or holes in the tubing makes it useless. Just be sure the tube is sealed and secure at both ends with clamps.
It sounds like you really need to start at the beginning and do a good Solid TUNE-UP on Your C4 just like suggested above.
First, do you have a Copy of the Factory Service Manuals? RockAuto has them on DVD for ~$35 and hard Copies can be had for about $85. This is a book set that you will NEED so get one if you don't have one yet. To tune up your L98 engine is more involved than replacing Plugs and wires like on older cars. The Throttle Body is where a lot of important things happen. There is Throttle Position Switch that tells the ECM the desired speed and it wears out. I would replace it with a good quality part and keep the old one. Then you have the Idle Air Control Valve which is a "controlled air leak" but can need attention. The IAC is what controls the idle speed in situations like yours, I would certainly look there before too long. When you buy a New IAC save the old one but be absolutely sure it has the same Pintle shape on both of them. NAPA tried selling me one with a newer type tip than my car came with. It seems that somewhere during the production year GM changed the Pintle Design to a "better" design.
I have a 1988 and have over 25 years with the beast. Have you got a Fuel Pressure Gauge on a long hose? They attach to the Schrader valve (on the passengers side fuel pressure line) and allow you to see the pressure changes while driving which can be helpful. How about any kind of OBD1 Scanner or even Code Reader? They are very handy and are able to save you a lot of time potentially. You want to be sure the Fuel pressure is within the recommended numbers, It is important to know that your Fuel Pressure Regulator is working properly. Find the device and remove it's vacuum hose and smell inside the hose, if you smell raw gasoline then there is a bad diaphragm in the FPR.
This engine operates in two basic modes. Open Loop is used when you first start the engine and it stays in Open loop until it either times out or starts getting a signal from the Oxygen sensor that should be warming up to operating temperature. Once the Oxygen Sensor starts sending it's oscillating signal the MAF and Coolant Temperature Sensor and O2 do "most" of the air/fuel calculations and this is called "Closed Loop" and in this mode the engine runs much cleaner and more efficiently as it is monitoring the O2 and adjusting the fuel accordingly. There are times that the car reverts back to Open Loop like wide open throttle for example, there are others. Even at long stop lights the older UN-Heated Oxygen sensors can cool down enough to force the engine back into "Open Loop". Then at a green light the L98 powered Corvette takes off and the O2 gets HOT fast and the engine switches back to Closed Loop again.
The Oxygen Sensor on your C4 was designed for a 24 month/50k usage and then it was replaced. They can and do work sometimes past their useful life but the signal gets narrower and slower over time. The New O2 will make your engine run more efficiently and you should have better performance depending on how old the original is. Besides they are cheap and easy to install.
Well Bottlejunkie you came to the right place for answers on how to "help" your Corvette! You are most Welcome to the Corvette Forum and we all look forward to hearing how your Corvette turns out!
It sounds like you really need to start at the beginning and do a good Solid TUNE-UP on Your C4 just like suggested above.
First, do you have a Copy of the Factory Service Manuals? RockAuto has them on DVD for ~$35 and hard Copies can be had for about $85. This is a book set that you will NEED so get one if you don't have one yet. To tune up your L98 engine is more involved than replacing Plugs and wires like on older cars. The Throttle Body is where a lot of important things happen. There is Throttle Position Switch that tells the ECM the desired speed and it wears out. I would replace it with a good quality part and keep the old one. Then you have the Idle Air Control Valve which is a "controlled air leak" but can need attention. The IAC is what controls the idle speed in situations like yours, I would certainly look there before too long. When you buy a New IAC save the old one but be absolutely sure it has the same Pintle shape on both of them. NAPA tried selling me one with a newer type tip than my car came with. It seems that somewhere during the production year GM changed the Pintle Design to a "better" design.
I have a 1988 and have over 25 years with the beast. Have you got a Fuel Pressure Gauge on a long hose? They attach to the Schrader valve (on the passengers side fuel pressure line) and allow you to see the pressure changes while driving which can be helpful. How about any kind of OBD1 Scanner or even Code Reader? They are very handy and are able to save you a lot of time potentially. You want to be sure the Fuel pressure is within the recommended numbers, It is important to know that your Fuel Pressure Regulator is working properly. Find the device and remove it's vacuum hose and smell inside the hose, if you smell raw gasoline then there is a bad diaphragm in the FPR.
This engine operates in two basic modes. Open Loop is used when you first start the engine and it stays in Open loop until it either times out or starts getting a signal from the Oxygen sensor that should be warming up to operating temperature. Once the Oxygen Sensor starts sending it's oscillating signal the MAF and Coolant Temperature Sensor and O2 do "most" of the air/fuel calculations and this is called "Closed Loop" and in this mode the engine runs much cleaner and more efficiently as it is monitoring the O2 and adjusting the fuel accordingly. There are times that the car reverts back to Open Loop like wide open throttle for example, there are others. Even at long stop lights the older UN-Heated Oxygen sensors can cool down enough to force the engine back into "Open Loop". Then at a green light the L98 powered Corvette takes off and the O2 gets HOT fast and the engine switches back to Closed Loop again.
The Oxygen Sensor on your C4 was designed for a 24 month/50k usage and then it was replaced. They can and do work sometimes past their useful life but the signal gets narrower and slower over time. The New O2 will make your engine run more efficiently and you should have better performance depending on how old the original is. Besides they are cheap and easy to install.
Well Bottlejunkie you came to the right place for answers on how to "help" your Corvette! You are most Welcome to the Corvette Forum and we all look forward to hearing how your Corvette turns out!
That was a great read and I learned a lot. I do not even have a Vette right now but looking at acquiring another one soon and just reading threads here.. I learned some things I did not know. Thanks.
Pro
Joined: Aug 2021
Posts: 503
Likes: 33
From: S Fla
Same here. I've got one I'm looking at, so I came here and started learning. The amount of people here who are willing to help, and the amount of info already available, is unreal. And very much appreciated.
Thread Starter
Melting Slicks
Joined: Nov 2021
Posts: 3,495
Likes: 16
From: Conroe Texas
2024 C4 of the Year Finalist - Unmodified
so I finally got around to pulling the air box today and found this.
looks like he chopped something off of it and also added a K&N filter, the car still does the rev up/down when stopped and the amber service engine light has come on but it goes back off.
looks like he chopped something off of it and also added a K&N filter, the car still does the rev up/down when stopped and the amber service engine light has come on but it goes back off.
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Pro
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From: Liliha Bakery stuffing my face with coco puffs!
Thread Starter
Melting Slicks
Joined: Nov 2021
Posts: 3,495
Likes: 16
From: Conroe Texas
2024 C4 of the Year Finalist - Unmodified
Pro
Joined: Aug 2020
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From: Liliha Bakery stuffing my face with coco puffs!
Drifting
Joined: Feb 2017
Posts: 1,271
Likes: 398
From: Tacoma WA
2024 C4 of the Year Finalist - Unmodified
Owners often “cut the lid” in an effort to generate more airflow is all that is being shown in your photo. It really doesn’t do much if anything for improving airflow but this is not something to worry about.
