Here's the latest issue with my idle surging. Replaced wires plugs, cap rotor yesterday. Has engine surge at any rpm except when I manually turn on the main fan. When I have the main fan running the car idles and runs beautiful. Any ideas?


​https://youtu.be/7F3i8gnnl4k

Video showing issue

 

How is that manual fan switch wired?

 

Green wire from fan relay to switch in car then grounded to ground on shifter light

 

Unfortunately, I can only reference a 1990 which is obviously not the same year, so wiring might be completely different. That being said, I looked through the schematics for the 1990 to see what else was near or in the same circuit. It looks like the control lines for both cooling fan relays run through connector C137. In 1990, another of the terminals in C137 was providing the ground connection for the cruise control solenoid as well. This is just pure specualation, but since something obviously strange is going on, I would guess that one of the circuits sharing that same ground distribution point (shown in my manual as S119) has broken - perhaps the ring lug to the frame at G106 broke, for example. When you flip your fan switch, you provide an alternate ground path and things go back to working correctly. I realize this is still a low-probability guess since I think it would also have to imply that a few of the terminals in C137 had also shorted to each other. I've put in a few images from my FSM below to show you what I'm talking about. Again, your wiring might be completely different since this year didn't even appear to have a green wire coming from the fan relay. Good luck.


Diagram of the cooling fans / relays.

Ground distribution section showing same connector as in cooling fan circuit.

Location in car of this shared connector (C137)

C137 terminals if you need to ring out the connector.

 

Is that your SES light flickering but seems to stay off with the fan on???

 

I have no idea if this might help but on the Corvettes built with the "Auxiallary Fan" like my 1988 C4 has have a second fan in front of the radiator. It is a factory installed cooling "supplement."
The switch that activates this Auxiallary Fan is a single wire that you ground to run. It attaches to the engine block on my 1988 C4 L98 it is between cylinders #1 and #3 and is a Green wire with a White stripe I thought. (Remember now I am Color Blind and getting old....) I think the relay for it is very accessible on the firewall.
It is fairly common for some us who live and operate primarily in hot weather with heavier traffic to switch this "On" for most of the cooling season. I let it run whenever the Corvette is being used as it pushes a decent amount of air across the condenser and the radiator. It is back-up and helps me not live "watching" the temperature gauge.

 

Well it looks like I figured it out when I had the AC running that is when I would have the issue sure enough I pulled out the AC fuse and it is purring like a kitten. I can live without AC just thankful to have finally figured it out after nearly going through the whole car several times. Thank you everyone for the suggestions.

 

Yes, I understand what you're getting at, but it doesn't help much. The schematic I posted for the 1990 also has two fans. The circuit you are grounding - I am guessing - is one side of the coil wire on the fan relay. It's not at all uncommon for circuits like this to be set up to complete the ground path rather than trying to switch the voltage on the "high side" since it's easier for solid state circuits to "sink" current to ground rather than "source" it from the positive. My guess from above is basically this: The reason things get better when you switch the fan to ground is that it's ALSO providing a ground to something else nearby that was somehow missing it. This would require some wires to be chafed and shorted together, broken, etc. Unfortunately, I don't have an FSM for the 1988 so I don't know that the locations of these things are the same. The reference material I posted was for the 1990 L98 and that's about as close as I can get.

 

Hello phaze426

Your schematics are right for the 1990 model and it should be the same you would think but not with GM. I have had this C4 for about 25 year and it like all L98's likes' to run warm. When I first bought the Corvette I noticed it had a bad crimp on the wire that goes to the temperature switch. After following it around I realized it was the easiest way to help keep the car from operating as hot as normal. I believe it is a B4P option code for the 1988 C4. It is also possible to buy lower temperature switches for this auxiliary fan.

Pulling things to ground is better than shoving 12 volts down the wires for the Computer. I love when people first get their Corvette and they find 12 volts at the injectors the whole time the engine is running. The injector coils fire when pulled to ground. This is a hard concept for some folks to grasp. You obviously have some experience with PLC's and know how they operate.

If you would like I can try and post a copy of the 1988 Schematic showing how the aux fan is wired into the system. You might learn more about it just googling B4P Option code for 1988 or better yet check it out on this Forum. It is strange because I am not used to seeing fans in front of the radiator. One in front and one behind, when they are both on there is a LOT of air being pushed through the radiator. Maybe it wasn't the greatest idea but it works.