whatdo

ive been around the internet and it looks like this forum has the best knowledge of tbi's, though mine is in a 1988 chevy k1500. hope you dont mind

when my truck is idling, it sputters and surges and has that throaty sound that you get with super retarded timing. when i hook up the timing light, i can see the timing mark jump around within 5 degrees of one spot (with set timing connector unplugged), so i cant really set the timing properly. with the set timing connector plugged in it doesnt advance nearly as much as it should, and the timing mark moves around even more.

i've got a new iac valve, new coolant temp sensor, new knock sensor, new distributor cap, rotor, plug wires and plugs, map sensor, and probably some more things that i cant remember

i took the distributor out to have a look and the reluctor magnet thing was completely crumbling, i figured that might have something to do with it, so i bought one of those new cardone select distributors. after installing the cardone distributor its even worse.

do i need to get a GM distributor? or is it something else? i'm completely out of ideas now. i've been riding my motorcycle to work in the meantime and its starting to get cold out here

dmccain

iTrader: (2)

Sometimes after you do everything in your power its just best to take it to a good shop

Schurkey

Connect a scan tool, look at the data stream.

Connect a fuel pressure gauge, tell us what the pressure is--preferably under heavy load, but even an idle test will be helpful.

whatdo

i ended up taking it to a chevy dealer since i dont have an aldl scan tool, the mechanic there said he thinks its the cam chain and that he ruled everything else out. i'm not really sure about that, i'm gonna open it up this weekend and find out

does this sound like it could possibly be the cam chain? it seems a bit weird to me that the cam chain would cause the timing to move around like it does

also how do you get at it on these motors? drop the oil pan?

Schurkey

How can he "THINK" it's the timing chain? Either it's tested good, or it's proven bad. Either way, he should have told you that
1. The fuel pressure is/is not adequate,
2. The codes and data stream showed nothing unexpected,
3. That the cranking compression is acceptable on all cylinders
4. The cylinder leakdown results are acceptable on all cylinders
5. The power balance tests are also acceptable.

Failing cam chain/sprockets is not out-of-the-question. Easy to verify: Put damper at TDC by carefully turning in the direction of normal crankshaft rotation (clockwise seen from the front) then remove distributor cap, watch rotor as the damper is turned backwards SLOWLY AND CAREFULLY. When the rotor begins to turn backwards, STOP turning the crank. Look on the timing pointer to see how many "degrees" of slop the chain/sprockets have. A couple of degrees is good, 5 degrees is acceptable but not good, 10 degrees is totally shot.

Worn distributor or camshaft distributor drive gears can interfere with this method--but that's also fairly rare.

You'll remove the accessory belt, water pump, timing cover; and you'll have to loosen the oil pan to get the timing cover off--and back on. If it was me, I'd pull the engine and re-seal/re-gasket the whole thing if the timing chain needs to be replaced. If the timing chain cam gear is of the nylon-tooth style, there's a good chance there's a bunch of teeth in the oil pan that should be removed before they plug the oil pump pickup screen.

whatdo

about 7 degrees of play there between the distributor and crank. if i grab the belt and turn the crank back and forth with it i can feel the looseness of the chain and where it catches on either side

gonna order the new chain/sprockets, might swap the cam too while i'm at it.

if this doesn't help i've ordered an aldl-usb cable which will be arriving soon, then i'll be able to know whats going on in the ecm

i wish i could pull the engine, just dont have the equipment. compression is good and consistent except for cylinder 8 which is 30-40psi lower than the rest. cracked ring probably.

Schurkey

First Guess: Driveability problems are not the result of the worn timing chain. Seven degrees of slop is not healthy, but I don't see it creating the problems you list in the first post--except for the unsteady timing.

OTOH, you know the chain is worn, and I've been wrong before.

whatdo

yeah, i didn't really think it made sense that a loose timing chain would make the engine behave like that, i'd more expect to just be noisy unless it had jumped a tooth

oddly this problem didnt start until i replaced the throttle body base gasket, and the vacuum line to the map sensor. prior to that it was idling at 1100rpm with the throttle screw all the way out because of the vacuum leaks and would sometimes stall out in drive but it idled reasonably and ran well and the timing didnt behave like it does now

i guess i'll find out next weekend when i've got the timing done and can see the data stream

whatdo

righto, brand new timing chain and sprockets are installed as well as some new distributor bits. timing mark doesnt move around anymore, runs better but the problem still isnt quite fixed.

found that the tps is at 42% and 0.82 with the throttle closed. at WOT it is like 220% and 4.1. all the other readings seem alright.