Hey Guys

Just want some opinions as to what you think this problem could be, and what I should be looking at to correct it?

I've asked the odd quick question about it before, but here's the full picture.


History:

When I got my vette ('86 4+3), you could turn the key, and it would idle at 600rpm on it's own, cold or warm, though it seemed to always fire easier from cold.


What happened:

One summer a couple of years back, I took it to a car show. Leaving the car show, I was forced to drive down a very bumpy (for a Corvette) gravel driveway, which I drove down as carefully as I could.

Upon rejoining traffic on normal road, minutes after leaving the driveway, my vette just stalled. I could fire it right back up, but it would just instantly stall again. If I kept my foot on the throttle, it would run, the moment I let go, stalled.

Cutting this story short, I wound up the throttle rest/stop a couple of turns to make the car idle a bit higher (around 700-800rpm) which stopped it from stalling while driving it.

Basically i've been operating my car like this since.


How it starts now:

If I turn the key alone, it stalls. If I hold the throttle on slightly for about a minute

(about when it hits 45C coolant temp), the car will increase in revs and 'take it' from me, then holding the idle by itself and allowing me to release the pedal without it stalling.



So basically, i'd just like to get it back to running like it did when I got it. I've had no other running issues apart from the idle.

Some have suggested checking out the IAC valve, or cleaning it/adjusting it?

I'm also wondering what role the TPS plays in the idling? How can I check that this is ok?

Is there anything else I should be checking?

Am I likely to have a faulty sensor somewhere? or shaken something loose when I went down that rough driveway?

I'm enlisting the help of my ex-mechanic father next week, but he's not too savvy on the computer side of things.


I will try and make a couple of videos tomorrow so you guys can see exactly what happens.


Thanks in advance!

 

Cleaning the IAC is a good thing to do every few years. You can't hurt it by cleaning. After that, you set the IAC with a scanner and then the TPS. Of course the timing should be spot on.

Do you have a scanner?

 

First pull your codes and see what that tells you. You should also Ohm your injectors and see if any are bad. Also, check your fuel pressure at the schrader valve on your passenger side fuel rail.

Your problems are most likely in the IAC motor, TPS setting, dirty throttle body, weak or dirty injectors, fuel pump, fuel pressure regulator, etc., etc. So before you start throwing expensive parts at it, pull your codes and start with following up on those findings.

 

I would check the fuel lines underneath the car for damage from the bumpy road.

 

No I don't, unfortunately

Is there a way to do things without one?


If the 'Service Engine Soon' / engine check light hasn't been lit, will it still produce codes? (just wondering?).

This is all new territory for me despite having owned my vette for a few years now, so forgive me if I sound like a n00b

 

Good advice, thanks very much. I'll need the newbie how to pull codes with a paperclip FAQ if there is one?

I'm guessing I need to buy a guage to put on the shrader?

My father's first suggestion was to clean the throttle body, but he was curious about how the TPS was linked to idle control. How can I test that the TPS is adjusted correctly?

 

I would get a scanner for the simple reason that even if we could fix this one, would you have another problem later on? Sure. It would make fixing it easier. If you miss the code at the time it happens and it corrects itself, it could have a history code.

 

http://www.extreme-check-engine-ligh...%20Decoder.htm

Yes put the gauge on the schraeder valve.

3 wires. One 5V, the other ground and the 3rd is the feed to the ECM. Get the ground and the feed and you have the TPS. With the engine off and the key on run, slowly open the throttle and see the voltage increase. It should be smooth with NO breaks.

 

First of all, if you have 1986 FSM, look up the TPS settings. Can't go wrong with the manufacturer's advice.

Second, here is a link that might shed some light if you don't have an '86 FSM...

http://forums.corvetteforum.com/c4-t...dle-speed.html


P.S. just saw Aklim's post-great chart!

 

Made a couple of videos so you guys can see exactly what it does.

Video 1:

Cold start, WITHOUT touching the accelerator.

To my surprise, it actually almost wanted to idle at 400rpm on it's own. It doesn't usually do this, so I turned it off and then started it again and it then did it's usual trick:

http://youtu.be/5poMej23qVg



Video 2:

Here i'm holding my foot gently on the accelerator, keeping it at 700rpm. Watch when it hits 40C coolant temp, the revs increase on their own - this is when I know I can release the pedal and it will idle on it's own. My foot did not move through this video until the end where I release it and it sits at 600rpm.

http://youtu.be/Xjid1m5jygU

 

I wasn't able to see the videos, but I had something similar going on late last year that had me beating my head on the wall. Turns out it was the MAF relays (yes there are two) on my 86L. Here's a thread for my thank you post to the forum. If you're bored, you can also look at my posts prior to this where I was trying to figure it out while trying to beat winter! It was a cheap and pretty easy to replace fix- once I finally narrowed it down! Good luck.

http://forums.corvetteforum.com/c4-g...af-relays.html

 

You need a $15 test harness which will make it very easy to set your TPS position. See the attached instructions and the DYI test harness below:

http://www.mamotorworks.com/corvette?frame=3.900

With regard to reading your codes, you can download free tuner software (WINALDL or TUNER PRO) and use the $65 ALDL cable to connect your laptop to your Corvette to pull & read your codes. THEN, use your factory service manual to properly trouble shoot & clear each reported trouble code.

 

Thanks for the info guys, will do some reading of the above and I can see i'm going to need to buy a few tools (which will take a while to get if I have to buy from USA). I've seen scanners for sale here in NZ, but how would I know if one is suitable for a vette?


Oh and is everyone able to view the videos ok?

 

You can build your own cable and use an older laptop with a serial port. Then use Datamaster or Tunerpro.
It cost me less than $5 to build my cable.



As for your idle, I haven't seen the vids yet, but I would try and set your minimum idle and TPS idle voltage.

http://www.lbfun.com/warehouse/tech_...ch%20Paper.pdf

I can slip my meter's leads under TPS's weatherpack rubber and along top of wire, until it reaches the metal pin, to get my TPS voltage. I have also used brad nails and paper clips, to get to connector pins, whatever works.

 

I was able to view the videos. My car did something similair to this, wich was caused by a failing fuel pump. That's really cool what you posted up, agent 86

 

Watched your videos, weird how at 40C it seems all good.
As mentioned, cleaning the IAC's pintle, seat and port, would be a good idea, then try and set you min idle and TPS.

 

If you or anyone with the 165 ECM, "E" is the data line, if you plan on building your own cable.
I stuffed all the components into the DB9 connector, not the easiest way.

 

In our case an OBD I scanner is fine. See if you can get it with a return policy

 

This is all good stuff guys, much appreciated. Will get to checking things out soon.

Always wondered about the OBD thing. So it's 'I' I need? a II won't do it?

I might see if I can find one for sale @ mid america or ecklers and buy while the dollar is good. It'd probably be in my best interest to have a scanner since I have no plans to sell the car

 

Your port might be different so IDK. My scanner is capable of OBD II and OBD I so I have different plugs.