Ok so I have this 1986 Corvette that I traded my Jeep for. It has the L98 with 700R4 Trans.

It is miss firing and running rough. Has no power at all when running. It will stall sometimes and you can tell it's not running on all cylinders.

I have changed the cap, rotor, wires and plugs. Checked the timing and all good. I have also checked air filter to make sure not clogged. It has new MAF relays and a new coil.

Still runs rough. Not sure what to do. What could be my problem? Top end was just rebuilt about a year ago.

What should I look for? Also my temp gauge keeps reading "Lo" It never changes. The coolant temp sensor that is in it the PO put in and it only has one wire. Is this correct?

What is the EST? Is that something that could be he problem?

Please help.

 

Have you checked the fuel pressure?Whats the reading?Injector ohms hot and cold?

The EST,have you unplugged the tan/black wire when setting the timing?
Its near the brake booster.Did you plug it back in after setting the base timing?That can throw a code,so youll have to clear the code by disconnecting the negative cable for a minute.

Are you getting a check engine light or codes?I ask since you mentioned the EST.

Also,did you know,using a timing light,you can see which wires are misfiring? I do this anytime I have misfires.I even use this on cars that dont use distributors.

I hook the timing light up to the battery and check each wire by clipping the sensor on each wire,and activate the light...

When the wires misfire they wont constantly flash,you will see dead pauses between the flashes,compared to a good cylinder that fires faster or slower with rpm/engine speeds.It helps to narrow down a misfire problem to the cylinder(s).

Also be sure the plug wires are all in the correct firing order.Some of these older engines,the balancer slips,so on the timing checks,they get thrown off.Be sure your balancer is GOOD,and hasnt slipped,putting your timing marks someplace else.

 

A low coolant temp can be a bad CTS sensor,or a busted or missing thermostat.
Ive seen this happen when someone removed the T stat or where one was busted,stuck wide open.Temps never came up to normal.Always stayed LO or pegged the low side and never came up.

Check those.

 

Have not checked the fuel pressure. Don't have a gauge. I have no idea how to check injector ohms. I did time it when I unplugged the EST but didn't seem to make a difference is why i'm asking. What does it do? I think there may be a problem there.

Yes I did unplug the tan/black wire near the booster.

Would MAF cause a miss fire? Or even a TPS?

Thanks

 

ok so I changed plugs wires cap and rotor. Also checked timing again. Throws no codes. Replaced maf relays today and still runs the same. Any ideas?

 

Suggest checking the compression to insure all 8 cylinders are working before you throw anymore parts at it.

 

When you were told the top end was rebuilt a year ago - what exactly did that entail? It is possible that the valves were not adjusted properly if the heads were involved in the top end. I'd figure out what is happening with the coolant temperature sensor and the lo temp reading - does it smell like it is running rich? Most important - check for codes as I can't imagine an engine running that bad without throwing a code or 2...

 

It is running rich. All the plugs were black when pulled.

I don't think it's a compression issue if all the plugs look the same right?

When i unplug the MAF it still runs the same. Think that may be the prob? I was told the MAF could still be bad w/o throwing a code. The coolant temp sensor that was changed is for the gauge not the fan. Would that one affect running of the motor or the one for the fan cuz that acts like a choke kinda. The top end was heads and up.

It runs but has no power and def getting to much fuel as the back of the car by exhaust is black.

 

Listen to what these guys are telling you about the CTS. It acts like the choke on a carb. If it is not working right, the engine will run very rich, to the point that if will flood. Ask me now I know. Make sure that the CTS does not have any teflon tape around it as a seal. This can cause it to give a false reading to the computer. There should be two wires that run to it. Look on the pass side head, between cylinders 6 and 8, and see if the temp sensor there is hooked up. It should have one wire. (These cars have two temp sensors, one for the computer and one for the gauge. The one for the computer is in the front of the manifold, under the thermostat housing.)

 

Buy yourself a fuel pressure gauge.

Any traces of fuel in the fuel reg vacuum hose ??


http://members.shaw.ca/corvette86/Fu...mDiagnosis.pdf

 

Get a fuel presure gage first. At the top of the tech is a sticky from FIC that will show you how to check your injectors.

 

checked my injectors and good. Fuel pressure is right at specs also. Is there just one o2 sensor on this thing?

 

Yes

 

ok i'm going to change that. Someone told me that could be bad and not throw a code. Is that possible?

 

I believe so, but to just throw part at these cars is nuts The 02 sensor is not a controled devise but it dose send a singal to the EMC. However I'm going from my 87 FSM

 

i know but I'm stumped. Has good compression plugs i just put in and cap and rotor were new as well as wires when i got it. Changed the plugs cuz they were a little black from running rich. Threw a code for MAF so put that on and now doesn't stall and no codes but still running rough. I mean i just don't understand it. Firing order was triple checked so i know that is good. I'm frustrated and wanna drive it and not sure what direction to go.

 

When you changed the MAF did you also change the relays. It can and will throw a MAF code because of other things in the MAF system. Do you have a FSM?

 

I suspect your system isn't going into Closed Loop operation. You can check using a paper clip in the ALDL and watch to see how quickly or slowly the SES dash light flashes.

You let the engine idle for seveal minutes because it takes that long for the system to normally go into Closed Loop; there's a timer that has to run out.

If it won't go into Closed Loop, and considering all you've done already, I'd suspect the 02 sensor is bad. The 02 sensor is critical and one of the main things the ECM uses feed-back from to make adjusments to fuel and timing.

You can scan for any stored codes using the same paper clip and counting the flashes. I suspect the procedure is posted here in Tech Tips.

A Scanner would be even better because it'll show exactly what's going on with the engine. On hard to find causes, they're worth their weight in gold. When I had an 86, that's how I found my 02 problem.

Hope this helps.

Jake

West Point ROCKS!

 

relays are new put them in with maf. Put CTS in as a new one came with car. Fuel pressure is at 40 and under load goes to about 35-40. If dist is off a tooth will that do this?

 

Didn't you write that you already set the initial timing with the EST disconnected?

Did you set it at 6 degress BTDC?

After setting the timing, did you shut down the engine, reconnect the EST connected, then disconnect the battery for about 30 seconds or so?

If you did, try reconnecting your timing light, fire the engine and use the timing light to see that the timing is advancing as you rev the engine.

Once the ECM on my 86 wasn't taking over the timing and the engine ran as yours is. I discovered the problem by swapping a friend's ECM, taken from his 86Vette. A hundred dollars later and a remanufactured ECM from O'Reilly's solved the problem.

Here, again, a scanner would have pin-pointed the issue.

Jake

West Point ROCKS!