Hi
I have read before that there is a wire to diconnect to return the car to base timing. Which wire and where is it located?
Also, here is my problem....
When I start the car cold, it fires up no problem and the idle come up to 1000, but when you step on it, it wants to hesitate and miss abit and the throttle seems like it is on an elastic. Hold the rpm at 1400 and rev it up it is responsive.
When the car warms up and idles at 670 it still hesitates a bit. Kind of elasticy on the pedal again. But at 1400 rpm and step on it, it is responsive.
I have cleaned the IAC and set the TPS to .6 Volts. New plugs, cap and rotor.
I am going to check timing. I also wonder if i have exhaust backpressure issues.
Any other suggestions? Tests? Injectors? MAF? Help!

Thanks

 

I am wondering about the exhaust too. What year?

The wire you are looking for is tan. It runs between the engine and the wiper motor, along the firewall, drivers side. It will have a connecter on it.

Disconnect said wire.

You will also need a wrench to loosen the distributor. The bolt is reached from the passengers side. Sears sells the right tool, 9/16" distributor wrench. It's long and bent.

You will also need a timing light. It's long and not bent. Actually, it looks like a radar gun!

connect the light to the + and - of the battery, and the inductive pickup to #1 plug. Set the gun on the tire and start car.

Pointing the gun down to the crank, you will be looking for a line passing the light blast. That line needs to line up with the 6* mark. Now, I don't know what year you have, and I think the marks changed a bit. I hope someone chimes in on that.

As you watch that line - the timing mark - pass, turn the distributor a little to move it up or down, so as to advance or retard timing.

Again, I need to know what year you have so I know what timing gauge you have... that would help a little.

 

Please always include YR and Engine (i.e.: 87/L98) in your posts, there are several C4 types :
1. disconnect the est wire located by the brake booster (L98 version).
2. Set timing - L98 stock 6BTDC
3. Reconnect est.

Injectors: Fuel pressure test (l98) 40-43 psi at schrader valve on injector rail.

Did you correctly set the IAC during installation?
Check plug wires for short or arch problems.
Maf: disconnect (if car runs better) problem, but could also be the relays located behind the battery on the fire-wall

 

Ohm your injectors, cold and hot. It should have an est wire located on the bundle of wires between the distributor and the brake booster. It might be on the bottom of the bundle. We had a hard time finding it. When disconnected is disables computer so you can set the base timing.

 

Sorry guys. Its an 85 with the L98 Auto
Yes, I set the IAC correctly first jumping the pins into diagnostic mode for 30 secs, disconnect IAC, pull jumper and start car and bring idle down to about 450 rpm. Then I reconnect IAC and adjust TPS to .6V.

So I should notice a difference when the car is running and I disconnect the wire?

 

hi jinxie
you mentioned setting t.p.s @ .6v, i normally set my t.p.s at .54 volts closed throttle(engine off).
cheers
shae

 

He's still within the tolerances there. .6 isn't bad.

 

Ya I tried to give it a bit more than .54 becasue I thought maybe the hesitation was coming from a low TPS reading. No difference.

 

While setting your TPS did you do the entire setting? .54 to forget the WOT voltage, but you can check it...you're supposed to (while not running) check to see if the voltage increases smoothly to WOT, any fall off could mean your TPS sensor is bad. Or a bad connection to it, which alot of guys have found.

 

Disconnect the EST connector to set timing while the engine
is running. Here is a link with pictures.

.

 

When you disconnect the EST (tan) wire to set timing the ECM will set a code that should be cleared when you are done with the timing chore.

 

Ok Guys, so far so good.
I checked the timing and it was at about 10 btdc. I figured that the way the car was acting 4 degrees wouldnt make a difference. Well it did. I set it to 6 degrees and it runs 95% better!

Before I set the timing, i warmed the car up to 150 degrees. I would suddenly open the throttle from the idle stop screw and stop and hold it in line with the throttle body mount bolt (about 1/2" throttle or so) (manually with my hand in engine compartment) The car would stumble, engine would "jump" and I made it backfire thru the intake a few times.

After I set the timing, I tried the same thing. Very little stumble from off idle to that same mount bolt. But still slight. No backfires (tried it 50 or so times!) But an ever so slight hesitation. Its like I catch the engine off guard! However, it is very hard to mimick with my foot on the pedal. When I put my foot into it from idle to full throttle, sometimes I can make it slightly hesitate. I dunno.....being too fussy now?

I reset my IAC/idle and tps again to compensate for anything I changed while changing the timing. The idle did drop some when I retarded the timing. Now it is back up.
Other fine tune suggestions?
I have another 85 as well with no mufflers, just straight pipes. Im thinking of opening up this one as well. Any disadvantages? When the cooling fan cuts in you cant hear the engine run. Thats just wrong!!

Thanks

 

HELP!

It still seems to have a hesitation and backfire"pop" out of the intake. I just fired it up in my shop (12C or 54F degrees) and let it run for a few minutes. I put it in gear(still 850 rpm) and put my foot hard into it. The engine went du du pop pop and stalled. Fired it up and tried again. Same, but didnt stall. It revs up Ok when you ease into the throttle, but under hard acceleration it hesitates and pops from the intake.
It rund better when warm, but I can still feel that the car has a shake or slight roughness to it. I fired up my other 85 that was parked behind it and put it in gear and to the mat. I smoked the tires out of the shop with no delay or hesitation.
Again....new plugs, cap, rotor, tps and set to .6V, cleaned IAC and TB, timing set to 6 degrees.
I am going to check the resistance on the injectors.
What else?
vinnies87 suggested disconnecting maf. Do I just pull the plug under the intake boot?

Thanks again for the help!!!

 

If you have another '85 grab the maf off of that one and pop 'er in and give her a crank.

Other things to check:
1 Ensure that the car doesn't only act up at low RPM
2 Check to ensure that you have "good" exaust flow with the engine running for a bit (clogged converter)
3 Check the fuel pressure at the rail with the car running and then when turn the key off and check to see how long the pressure stays up.
4 Ohm the injectors out to see if they are within spec.

 

I would be inclined to make sure the distrubitor is in correctly. Take the #1 spark plug out, hold your finger over the hole, and bump the engine over until the pressure blows your finger off the hole. Look at the timing mark on the HB. It should be at 0*. Take the cap off the dist and make sure the rotor is pointing toward #1 cylinder or at the left rear point on the plenum. It's a fairly quick procedure and might help rule that out in the troubleshooting.

Make sure the spark plug wires are routed and placed correctly. There is a picture in the FSM.

 

Hi
Yes, I actually tried swapping the MAF's and no difference. Not sure exactly how the MAF system works with the burn off relays. Could it be something else in that circuit?
When it is warm is has a slight delay/hesitation. Definatley worse when cold. Its almost like fuel starvation. Like an old carb with too small of shooter sizes or accel pump. Pops/backfires slightly in intake.
I still need to check FP and resistance. I might even drop the exhaust?
If I hold the idle at 1200 rpm and quickly accelerate it is responsive and working 100%! Even cold. But any quick throttle action below 1200 has a hesitation.
I do have another perfectly running 85 Vette here. If there are other components, relays, sensors that I should try, let me know!
Thanks again

 

Sorry I didn't see this stated, but is the check engine light on? Of not, I would start checking things that would not cause a code. The popping through the intake, hesitation, rough idling, and shaking is exactly what mine did when the dist was installed wrong. When #1 cylinder was TDC the rotor was pointing toward #5 cylinder, IIRC. Pulled the dist and installed it correctly and all the problems when away. This is just a quick way to figure things out with miminal tools or knowledge.

Check the fuel pressure and leak down, ohm the injectors, check for clogged cats, ect also.

Have you had this car long or did you just purchase it?

 

 

Sorry for the delay.....

I just tested all the injectors...they are all reading 15.9 ohms. I havent had this car long and it has been running like this since I purchased it. I thought it would be a simple plugs, cap and rotor tune up. Changing those parts never made a difference.
I still havent checked the fuel pressure....have to buy a gauge.
When the car is running there is just a slight miss to it, but not like 1 cyl missing, but them all not running 100%. All 8 are firing, but the engine has a definate shake to it. Again, when it is first started it wont rev right up under fast acceleration...it just has a huge hessitation. when it is warm it hesitates less, but is still noticeable.
Maybe coil or pickup coil?

 

Jinxie: Have you set or adjusted the minimum air/base idle on your car?
This may help your problem.