I'm trying to figure out a problem with an '85 2.8 V6.
What does a loose timing chain act like?
Any ways to diagnose this short of traring the front of the engine down?
Thanks...
BobC

quote Originally posted by Cozmo: I'm trying to figure out a problem with an '85 2.8 V6. What does a loose timing chain act like? Any ways to diagnose this short of traring the front of the engine down? Thanks... BobC

Symptoms would be a crappy idle after clearing up any fuel control or ignition problems. When shooting base timing the mark will randomly jump more than +/- 5 degrees. Unfortunately, this can also be a crappy distributor bushing. If you have access to a scope, this can be seen watching the spark events line up (or not) in a stack pattern. To confirm, remove the distributor cap and put a wrench on the crankshaft bolt. Turn the crank backward until the distributor rotor moves. Note the position of the crank pulley. Turn the crank forward until the distributor barely starts moving. The difference in where the crank is now and the last position shows how many degrees of "slop" is in the chain.

EXCELLENT ADVICE !!!!!!!! You are a fart smeller......ahhhh I mean smart fella...LOL. Just kidding......That is prob the best most perfect advice in my viewpoint that I've seen on this forum! Congrats

Phil

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GTDude
25 years GM experience

I was going to compliment you on your suggestion but GT dude beat me to it. ITS JUST TOO BAD I DONT HAVE A DISTRIBUTOR. Otherwise Id go check mine out now...

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Matt D
1984 Indy Fiero 4sp.
1985 2m4 4T60, DIS V6 3.1
Soon to be, 3800 with a...

WOW!!!
Thanks, that's great information thomas_l!!
The fuel and spark stuff has been gone thru with no real improvement.
The car idles a little rough, and with no load stutters at anything above 2000RPM. If you give it some gas in gear, it smooths out and runs fine.
I'll do as you suggested and post what kind of "slop" I find...
Thanks again...
BobC

Give it a GOOD tune-up lately?
Cap, rotor, wires and plugs are a good tune-up along with filters and PVC valve.

I did a full tuneup (plugs, cap, wires, rotor, PCV), oil change and coolant flush last weekend, no real difference in the way it runs. I've also disconnected the tach filter, EGR vacuum line and cold start injector and swapped the coil and dist. from a good running engine with no success.
When I set the timing, I don't remember the timing mark jumping around very much.
If I put the timing light on the coil wire, than I'll see both cyls 1 and 4 on the timing mark... if there's a real difference than there's something hosed in the timing chain/ distributor, right?
I don't have access to a scope, but if I can't fix this soom I'll go scare one up.
This weekend I'll try the turning the crank nut while watching the dist. rotor as well as check everything else again...
This problem is starting to bug me, a real challenge...

Thanks to all!

BobC

quote Originally posted by Cozmo: I did a full tuneup (plugs, cap, wires, rotor, PCV), oil change and coolant flush last weekend, no real difference in the way it runs. I've also disconnected the tach filter, EGR vacuum line and cold start injector and swapped the coil and dist. from a good running engine with no success. When I set the timing, I don't remember the timing mark jumping around very much. If I put the timing light on the coil wire, than I'll see both cyls 1 and 4 on the timing mark... if there's a real difference than there's something hosed in the timing chain/ distributor, right? I don't have access to a scope, but if I can't fix this soom I'll go scare one up. This weekend I'll try the turning the crank nut while watching the dist. rotor as well as check everything else again... This problem is starting to bug me, a real challenge... Thanks to all! BobC

First of all thanks for the positive feedback . I think that's the first time in nearly 10 years of posting tech advice (on newsgroups before www came along). Makes me not so hesitant to participate and affirms that 5 years in a cushy office job hasn't caused me to forget everything .

Second - I wouldn't waste any more time trying to avoid using a wrench to check timing chain slop. Just do it because your next steps if you rule that out start getting tedious and maybe expensive. You have done a great job of covering the simple stuff - I especially liked the sub of a known good distributor.

What you are describing with your recent post really sounds like a lean fuel condition. Problem going away under load kind of rules out low fuel pressure and exhaust restrictions. Could be a leaky EGR system. Pulling the hose just keeps the valve from opening. You can only confirm by blocking it off. If your hot idle is less than 1000 rpm I would skip the EGR system - taking that rusty crap apart will probably lead to more work. Could also be dirty or clogged fuel injectors or leaky injector o-rings. Once again you can sorta support this on a scope (lean spark burn event) and maybe narrow it down to specific cylinders, but you'll probably end up pulling the rail and at least replacing the o-rings and getting the injectors tested somewhere anyway.