Aaron's-coupe

I need help to figure out why my car is running so bad, it is a 1986 manual with a 4+3. So quick back story, I am building a race car (the build thread is here https://www.corvetteforum.com/forums...durance-2.html) and we are having issues with the car running properly.

The problem: when you crank the car it starts up then is running real rich, then when you start to give it throttle it starts to bog, go lean and spits and runs like crap.

Here is what has been done so far… First the chip was re programed to remove the VATS, EGR Air pump. We have also replaced the IAT (the temp sensor in front of the manifold below the TB), replaced the IAC, set the TPS to .56v, set the timing to 6deg. I have also gone with and without the MAF hooked up and no difference.

I am at a loss on what to do next. Any help would be great.

Thanks

antfarmer2

I would look at your digital temp sensor connection and a leaky fpr diaphram. And call Jon at FIC.

billschroeder5842

AF2 is on to something.. that is the first place that I'd start.

Aaron's-coupe

i have replaced the coolant temp sensor (cts) I called it the wrong name in my original post.

I also forgot to mention that I have a fuel pressure gauge hooked up to the fuel rail and it reads 45psi.

aklim

When the engine idles, what does it read? Take the hose off and see what it reads. Finally shut the engine off and turn it back on but don't crank. Pressure should shoot up to 45 and hold for a while.

93Rubie

Why did you replace those sensors? Did they read/test bad?

Do you have any codes?

Do you have a scan tool to view the ECM data from the sensors?

Aaron's-coupe

So we messed with the car again today. I checked the fuel pressure, at key on, idle and power off it was always 45lbs.

One issue today I had was the car would crank, fire then stall. If you crack the throttle a tinny bit she would start right up. But again if you give it gas quickly it stumbles and wants to stall.

There are no codes other than 12... I don't have a scan tool.

aklim

I'd get a scanner or diagnostic work is going to be more guesswork than anything

Bwbob

I will 2nd on the scan tool but i would also hold the MAF highly suspect. Bad signal from that and the computer goes nuts. I had MAF power and burn-off relays messed up and it did similar things like yours. I did have codes though but it was a pile of them all over the place. I bought. Reman ECM and custom prom without AIR, VATS, EGR and reduced fans and a MAF but in the end it was faulty wiring in the MAF circuit

aklim

He did say it was run with and without the MAF connected so the only other way is to get another MAF and see if it does the same thing.

Bwbob

Mine ran about the same with the MAF connected or diconnected as well, kinda. Mine would start without the MAF and would idle but if i cravked the throttle fast it would just die. If i bumped the throttle real slow it would go to about 1500 rpm max then fall flat.
In my case the MAF was okit was a wiring issue from a previous owner or shop

Tom454

Having same problem.... can you be more specific what the wiring issue was?

Bwbob

On mine, the 2 relays behind the battery for the MAF have tye same exact plug but the relays and the plugs are wired differently. They were swapped. Im pretty sure the lot i bought it from knew there was an issue but couldnt figure it out. It seems that it criss wired something that affected the output from the sensor. Funny thing was it didnt mess with everything. I went round and round with the scan tool because i couldnt get it to connect so i figured my ECM was bad. A buddy had an early 86 and the scan tool worked fine. I could put my ECM in his car and it worked fine. I put his ecm in mine and same story but i could connect to it with the scan tool. His ECM was actually different inside but the plugs matched up. Finally i tryed setting the scan tool for an 87 and it connected. Go figure, i must have a really late 86.
Along the way i did find issues with several places where the insulation at various plugs had receeded back from the plug and were potentially shorting out.
I am assuming this problem showed up on yours all by itself but it does bear asking if any work was done recently.
Either way, i would inspect the wiring carefully.
I went as far a ohming out stuff from the computer to the sensors. When i hit the burn off circuit things got wierd and led me to the relays, which i tested good but never imagined the plugs could be swapped.

antfarmer2

Look at your TPS

aklim

Good point but we had a TPS that seems to be flaked out BUT when it is warmer. Found it out when we scanned during drive time after it got hot.

Tom454

How can you tell which plug goes to which?

Aaron's-coupe

Thanks guys. This car had some hacked up wires along with some other questionable repairs /mods on this car. The wiring for the MAF relay and burn off relay was all crumbling and broken along with some previous splices. By fixing the power feed to the MAF it seams to have helped resolve a lot of the issues.

Bwbob

Aaron's-coupe

So yesterday I got to work on the car. We got to properly re-wire the MAF and a couple loose ends. The car runs a lot better now bet we do have one smaller issue. Now when idling the rpm go from 500 to 1000 (up and down).

aklim

Assuming your timing is correct, have you tried resetting the idle by NOT using the FSM method and using a scanner? It should reveal if there is a leak somewhere.