Cannot get 87 coupe to idle correctly after head gasket replacement
 

I need help!

I have an 87 coupe that I just completed a rebuild of the top-end of the engine as a result of a blown head gasket.

As part of the work, we disassembled, cleaned, and redid pretty much everything we touched - diassembled and cleaned the heads, replaced valve guides, re-did all of the gaskets, disassembled and replaced the fuel injector filters and o-rings..we even replaced some of the old vacuum lines.

The engine is now reassembled and it DOES START. However, we cannot get the car to idle correctly.

If we manually manipulate the throttle to run the car at about 2000 RPM, the car seems to run ok. However, if left to idle normally, it will die within a second or two.

Where are we going wrong? We bought a distributor wrench thinking we needed to adjust the distributor somehow and that doesn't seem to do any good. Since we can't get it to idle, we can't check timing, etc.

Please help with ideas to diagnose this almost fixed car!
-David

Reset TPS and IAC that control idle?

Basics...
whats the fuel pressure? (pump, filter, screen/sock, return, regulator,??)
How do you know IF the injectors are good and operating? (ohms & noid)
has it been on a timing lite? (base timing)
Are the MAF relays good? (at least maf power)
TPS adjusted?
IAC self adjust
BLM resets after min 30 mph cruise for min 8 seconds min temp in CL.

You need to know the answers to just about ALL of those questions before you can start to diagnose an EFI engine.:yesnod:

Thank you for the tips - I do not know the answers to any of those questions...

In the service manual, there is a discussion about setting the TPS sensor, but it refers to verifying the voltage of the sensor with a scan tool - we don't have a way to do this. Is there a particular ohm value we should get on the Throttle position sensor? We know how to measure that with a voltmeter.

IAC? No idea how to check or set that.

No "check engine light" is on.

Please advise further, and I will start checking things and report results. We are kind of novices at this...

http://forums.corvetteforum.com/1574750414-post10.html

http://www.thirdgen.org/techboard/po...610-post2.html


There is no feedback from IAC if it is set wrong

Remove the injector fuses and check timing while you crank it with starter, that should get you close.
If you didn't mess with the TPS and min idle screw, they should be as you left it, before top end rebuild.

TPS voltage can be read with a volt meter - just probe the back of the plug while its still connected. Your looking for around 0.5v at idle and 4+v at WOT.

I don't beleive that IAC procedure for one minuite.

I should add that the EST bypass connector should be unplugged when setting timing.
Do you know where it is ?

Don't need a scan tool to set TPS voltage. Read this thread that describes setting the basic timing, idle and TPS voltage. There are no "adjustments" to make on the IAC. The ECM will set it.

Also re-check the vacuum lines; even if they are new, it's always possible that one has come off. There is a vacuum line at the base of the IAC housing. Is it on tight? The steel tube that the vacuum line attaches to is simply pressed into the housing; is it loose?

agreed - if you didn't remove the TPS or IAC your issues most likely lay else where.

Big vac leak is a chance.

you could try spraying a starter spray around the intake to see if the RPM's change - this would give you a clue as to where to look for with a Vac leak.

We did remove BOTH the TPS and IAC sensors from the throttle body when the car was apart to clean and repaint the outside of the throttle body.

Is the assumption that I did not reinstall them correctly and the adjustment issue could be what is making the car not remain running at idle?

Your going to have to reset the TPS idle voltage. The top 2 wires on TPS are the ones to test. .54v +- .075

The tps is touchy. I don't think you could just also it on and have it perfect. Some adjustment is in order

So leave the IAC sensor alone for now - all we did was unscrew it and screw it back in.

Just worry about testing the voltage on the TPS sensor?

Also, do I do this adjustment on the TPS sensor with key on, engine off, or what?

If you read the link posted by both rodj and myself, the procedures for setting TPS voltage are in there.:thumbs:

You measure the voltage on the TPS with the ignition on, engine not running. You can measure it with the engine running if you want, but it's just easier without a lot of heat and moving parts near where you're working.

The easiest way to do it is to stick a straight pin alongside the wire going into the center of the TPS connector. You need to work it into the connector and wiggle it around until it goes in and makes a connection with the contact inside the connector. The top wire goes to ground so you can just touch any metal part of the engine with one lead of the voltmeter. Set the voltmeter (I'm assuming you're using a DVM (digital voltmeter)) to the 2V range and touch the pin to measure the voltage. With a DVM it doesn't matter which lead you use. The electrons don't care what color the insulation is. If the leads are "backwards" you will just get a negative voltage reading -- no problem. It's the number that's important, not the polarity.

Loosen the TPS screws and move it until you measure close to 0.54 volts (the FSM says +/- .075 volts, which is between 0.465 and 0.615 volts -- I go for 0.53 to 0.55 myself). This value is not really critical because the ECM remembers the value it sees when you first turn on the ignition and uses that to determine what it calls "idle". Tighten the screws and recheck to see if the voltage has changed. Sometimes tightening the screws will tweak the TPS position.

Some people will tell you to open the throttle fully and check the voltage, saying that it should be more than some arbitrary value they made up, like 4.5 volts. If you don't get this voltage you're supposed to futz with the TPS position until you do. This is not necessary, because the ECM considers any voltage over 70% of the reference voltage (5 volts) to be WOT. 70% of 5 volts works out to 3.5 volts, so anything over 3.5 volts is OK.

Something just occured to me from another post..
You probably have some sensor connected to your throttle body and
you also have a coolant line passing through it.
It might have a lot to do with your rough idle problem if there is no coolant passing through that throttle body.
I could be wrong, but check to ensure you don't have any air pockets remaining in your coolant system or your sensors could be getting the wrong information and adding to the rough idle problem if not being the cause.