poor running 84

[geekinavette]

 

[G Winter]

Can anyone shed some light as to why the gap was closed up on the vettes?
Does it have something to do with combustion chamber design ?


Glenn

 

[chevyagain]

bill,
just thought to say hello, and thanks for all the info. where i stand now
set the timing you were right timing was set with that plug connected
now im set at 6 btdc. runs better but not 100%, parts replaced idle control
valves, 02 sensor, map sensor, knock sensor, fuel filter, coil, wires, cap,
rotor. only problem left is very high idle in park, oh, also checked vacuum
hoses. thought of drilling out those idle stops. but looks like if i back
those down, it still wont bring idle down enough. any thoughts
thank you--------------john

 

[scooter18155]

what are you considering a high idle? it will idle high untill the car warms up and then it should drop down to around 600-800 at least mine does and what i understand is normal. make sure you let the car run long enough to warm up and see where the idle is if you havent done so already. if it is warmed up and your above 1000 rpms still that would be abnormal.

 

[chevyagain]

ran long enough to drop idle, stiil way high, hey im from ny just moved to
virginia, guess i have to do the plenum gasket. see if you have a vacuum line coming from the front of the egr solenoid. service manual doesnt show one but theres a clip for one otherwise i just have two going to the solenoid, just wanna make sure--------------- john

 

[geekinavette]

ran long enough to drop idle, stiil way high, hey im from ny just moved to
virginia, guess i have to do the plenum gasket. see if you have a vacuum line coming from the front of the egr solenoid. service manual doesnt show one but theres a clip for one otherwise i just have two going to the solenoid, just wanna make sure--------------- john

What RPM is it idling at when warm? You could still have a vacuum leak at the plenum lid, and or throttle bodies that are out of balance. If they have ever been removed then that's a good possibility. Also check for "slop" in the throttle plate shafts...the holes in the TBs tend to wear out, and air can leak by there.

It's not terribly difficult to do a TB balance, you can make up a water manometer with a piece of wood and a length of clear plastic tubing. Or you can find someone local that knows how to do it...just make SURE they have done a crossfire TB balance specifically...it's a different animal.

Also check your TPS adjustment...for good measure.

Check out your Coolant Temp Sensor also...the one in the front of the intake tucked up underneat the AIR valve. If it's a single-wire version you should replace that with the 2-wire version (the extra black wire goes to ground). The 1-wire versions are rather unreliable.

That port on the front side of the EGR solenoid is just a vent, nothing connects to it.

Bill

 

[wishuwerehere82]

bill,
just thought to say hello, and thanks for all the info. where i stand now
set the timing you were right timing was set with that plug connected
now im set at 6 btdc. runs better but not 100%, parts replaced idle control
valves, 02 sensor, map sensor, knock sensor, fuel filter, coil, wires, cap,
rotor. only problem left is very high idle in park, oh, also checked vacuum
hoses. thought of drilling out those idle stops. but looks like if i back
those down, it still wont bring idle down enough. any thoughts
thank you--------------john

The idle screws are the only thing that will control the idle speed.
If you haven't drilled out the caps, you must do it! Most specifically, the rear TB controls the idle speed. The front one is linked by the balancing screw on the front TB linkage.
Here's an illustrated procedure to guide you.

http://www.corvettefever.com/techarticles/4688_chevrolet_corvette_efi_system_tuning/

 

[geekinavette]

The idle screws are the only thing that will control the idle speed.

I disagree...a vacuum leak can most definitely cause an increase in idle speed! The IAC valves also control idle speed.



Bill

 

[wishuwerehere82]

I disagree...a vacuum leak can most definitely cause an increase in idle speed! The IAC valves also control idle speed.



Bill

Heh, heh. We haven't disagreed on anything before, Bill. So this is a first!
OK, so let me ask you this. What adjustment would you change to purposely increase of decrease the idle speed? The idle speed stop, right?
Woops, gotta go, Lightning hits real close.....

 

[geekinavette]

Heh, heh. We haven't disagreed on anything before, Bill. So this is a first!
OK, so let me ask you this. What adjustment would you change to purposely increase of decrease the idle speed? The idle speed stop, right?
Woops, gotta go, Lightning hits real close.....

Hey nuthin wrong with a little respectful disagreement! heheh

I think the distinction we need to make here is a properly running engine vs. one with some sort of mechanical problem. I guess the point I'm making is that if my engine suddenly started idling at 1500RPM I would NOT immediately go to adjust the idle screw to bring it down!

Now, barring any vacuum leaks, or malfunctioning IAC valves, or anything like that...if I wanted to change my idle I'd go to the adjustment screw!

Also keep in mind that the EPROM has an idle setting in it...if the adjustment screw is set below that point then the ECM will keep the idle at the programmed setting via the IAC valves. Hence, the idle stop screw should ONLY be adjusted with the IAC valves CLOSED! The IAC valves should only be affecting idle when in gear (for automatics), and when the AC is on.

Bottom line...the idle stop screw is NOT totally independant, there ARE other things that can effect idle speed.

Stay away from that lightning bro!



Bill

 

[wishuwerehere82]

Hey nuthin wrong with a little respectful disagreement! heheh

I think the distinction we need to make here is a properly running engine vs. one with some sort of mechanical problem. I guess the point I'm making is that if my engine suddenly started idling at 1500RPM I would NOT immediately go to adjust the idle screw to bring it down!

Now, barring any vacuum leaks, or malfunctioning IAC valves, or anything like that...if I wanted to change my idle I'd go to the adjustment screw!

Also keep in mind that the EPROM has an idle setting in it...if the adjustment screw is set below that point then the ECM will keep the idle at the programmed setting via the IAC valves. Hence, the idle stop screw should ONLY be adjusted with the IAC valves CLOSED! The IAC valves should only be affecting idle when in gear (for automatics), and when the AC is on.

Bottom line...the idle stop screw is NOT totally independant, there ARE other things that can effect idle speed.

Stay away from that lightning bro!



Bill

Yep. you're right, the IAC's and the ECM will modify the idle speed, assuming all the settings on the timing, TB's and TPS are set right. But the idle screws will need to be accessable in order to balance the TB's and set the TPS initially.
So since the timing has been reset, and everything that was disconnected has been replaced, a re-tune is the next logical step towards getting Chevyagain's engine running back to specifications.

 

[geekinavette]

Hey Pete guess what...

I AGREE! :L

If it were me I would do a complete re-balance and idle adjust...indeed. Given how the timing was set before...who knows what kind of shape the TB adjustments are in!

Bill

 

[chevyagain]

thanks guys for all you help, but im about to give up, all i want is my old
283 57 chevy that i set the points with a match book cover. drilled out
caps but have not gone any further, feel like ripping out this crossfire
stuff and going to new intake and one four barrel. started car twice today idle well over 1500, hard starting, stalling, backfire through throttle bodies and exhaust has 85k miles, certified. so not too bad. ive been contemplating removal of plenum and re-gasket, thats my only hope
then set up throttle bodies. has anyone gone with a new setup
eliminating that plenum, i understand about a new kickdown cable for tranny that would sort of eliminate computer i guess

 

[wishuwerehere82]

That which does not kill us makes us stronger.
Friedrich Nietzsche
Stick with it. You will get it fixed..

 

[chevyagain]

thanks buddy maybe your right, sometimes we go way overboard
sleepless nights and all. then all of a sudden its the most simple
thing to fix. didnt mention this is an 84 convertible, which they
didnt make, a greenwood conversion, so ill pursue

 

[geekinavette]

It DOES get frustrating! But man let me tell ya...when you take a car like that...that has been mistreated previously, and get it running smoothly...there just is no greater satisfaction!

Hang in there with it...ESPECIALLY if you have a convertible conversion? WOW...PICTURES PLEASE!!!

You have a massive vacuum leak...somewhere. Been there. When mine was doing what you describe it was the EGR valve. Might check that...remove it and see if it's leaking, and clean it out while you're at it. They are notorious for building up carbon and sticking open...instant vacuum leak.

Re-sealing the plenum lid is NOT a bad idea by any means. I would just plan on going over the entire top end...from the lid up. Lid gasket, EGR, balancing...the works.

Bill

 

[chevyagain]

well i guess i have to hang in there, will try to get pics on, but my browser
doesnt work to good. i do know the feeling of having met the challange
you guys never gave up, so count me in--------thanks-----john

 

[chevyagain]

bill,
went ahead and changed EGR, no difference, bad backfire through
rear throttle body today. no alternative, remove top plenum and
start over. getting back to a regular intake and a good carb
other than kick down cable for tranny. what seems to be the bad idea about this-----------------------john

 

[wishuwerehere82]

I hate to say this, really....
But the biggest problem with getting a Vette that isn't running right is trying to find all the problems that may have been caused by the previous owner's lack of understanding of the system or having done something that affected it to the point of not running properly.
The fact is you just don't know the history of the mechanics that have "laid hands" on your engine. Heaven forbid some dealer mechanics tried to fix it.

The basic principles of the engine are fuel, spark, and timing.
But... the mechanical systems that are involved are the intake system, the valve train, the cam and lifters, and the spark control system. A misadjustment in any of these systems will result in a poorly running engine.

For an engine to be backfiring through the throttle bodies, there has to be a severe timing issue. OR a valve problem that is causing one or more of the intake valves to be open when the spark plug fires. Or, a spark plug wire is in the wrong place. (Wouldn't be the first time.)

Here's where I would start to focus on, rather than assuming that everything is OK except the crossfire injection system.

1) Do a compression test on all the cylinders. This will tell you if all the valves are seating properly in the valve seats. Who knows? Maybe the backlash on the lifters is too tight, and one of the intake valves never closes. One of your valves may be sticking open or burned.
Who knows? Maybe one of your exhaust lobes on the cam are completely rounded off and one of your exhaust valves never opens
(Mine had two bad exhaust lobes completely rounded off and never opened the valves.)( A dealer had just installed a new intake manifold gasket to "fix" the problem.)

2) Check your lifter preload adjustments. Pull the valve covers and reset your lifters to 1/2 to 3/4 turn of the rocker arm nut. You need a shop manual for the proper adjustment procedure. It will tell you where to position the crank and which valves to adjust at what time. And then measure the lift of the rocker arms. It will tell you whether the cam is good, or if the lobes are rounded off.
And yes, it sucks that you have to remove the A/C brackets to get the valve covers off the passenger side.

3) Assuming everthing checks out with 1 & 2, then you can go about the business of getting the crossfire system re-tuned and the TB's re-balanced.

Remember:
BUBBA is not our friend!
Bubba already disconnected your knock sensor, and who knows what else he did.
Now the good news, is that if you replace the crossfire system with a single carb, you will have exactly the same problem if there's a valve train issue..

Getting a good running engine is about the basics. Make sure the basics are checked out, then we can help you with the more exotic stuff.

Oh, and if you need help with a complete teardown and rebuild, we can do that too. We have been there, done that.

 

[chevyagain]

pete,
thanks for gettin back, i know for a fact new wires, cap and rotor
plus plugs were put in before i got it. my wires are too long to begin
with and not neat, i should not have overlooked this at the beginning. as for the backfire, i can agree with the valve issue. but i still have a gut feeling its ignition or plenum leak. maybe start in the dark and take
a look, who knows,have to rewire anyway