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Topic Review (Newest First)
07-23-2012 09:48 PM
Pre-Tuner I think the vacuum pump will tell you if it has a hole in the diaphragm but not if it's not closing all the way. I think you can remove it and clean out the carbon though.
07-23-2012 07:17 PM
alabamasho I'll double check in couple days, post results. i believe you can use vacuum pump on it it too. biggest problem is the stall out when I punch the gas a little. Thanks everyone for all the input!!
07-23-2012 05:44 PM
Pre-Tuner I don't know why I didn't think of it before but is the EGR valve sticking open a little? That can also cause a rough idle. With it cold and off, put a few fingers in the back of it and actuate the valve a little bit. Then start it and see if it changed at all. EGR is essentially a variable vacuum leak to the exhaust. Not sure how else to check it other than that.
07-23-2012 01:31 PM
alabamasho thanks Bogie. truck just has 138k on it, that's practically brand new down here in Bamer,, had 2 or 3 350's before with around 300k, and ran great. This one set up with an ole man few years, tank was rather nasty was reason I changed it. I've put 20k on it and had the slight miss. Gonna go ahead change the dizzy, which I think will fix. If not, compression test and vacuum, then make yard ornament cause I put a goodlooking stamped billet grill in it
07-23-2012 12:53 PM
Originally Posted by alabamasho View Post
searched most threads, still can't determine how to seat distributor completely. I'm at TDC with no alignement marks on distr. shaft.. reason I pulled, had missfire, no acceleration. and I've installed new tank, filters and TBI. still have same problems. detected no vacuum leaks spraying around TBI, no CEL on. removed distr. cap and rotor is pointing straight ahead down center of intake, instead of pointing toward no. 1 cylinder. do I ccw oil pump shaft, so rotor, when seated is pointed toward cyl #1? thanks, ya'll are the best!!
Given this is a 1991 it probably has a lot of miles on it, the signs of miss fire, and lack of power are also indicators that the timing chain and gears are wearing out. Typically the cam falls retarded when this happens this results in the distributor falling behind which makes for a common sign that the timing set is wearing out as the ignition timeing needs to be reset, essentially advancing the timing just to keep it the standard marks. Since the distributor is gear driven off the cam this is a common situation as the engine ages.

The only really good test for this is piston positive stop and a degree wheel. The testing is almost as much effort as simply replacing the timing set. A rough indicator that can be seen if the engine runs is to hook up a timing light, accelerate the engine a few thousand RPM then slam the throttle shut while observing the timing marks with the thiming light. If the timing mark moves all over the place it is one possible sign that that the timing set is worn out allowing the timing to run in and out at the chain rattles through the clearances between its links and their pins and its links between the gear teeth.

07-23-2012 12:46 PM
alabamasho I think that's my problem, both injectors are pulsating with the firing. Gonna yank the dizzy out. Hard to believe bad compression or springs. Had one valve cover off, motor is clean as a pen, don't burn oil or nothing. Clean lil truck inside and out. thanks everyone, take me couple days to get back, I'll let ya'll know how it goes!!
07-23-2012 12:26 PM
Pre-Tuner The injectors are also fired from the pickup coil. Take a look at the spray with a timing light and the engine running. Should be smooth on each injector.
07-23-2012 11:44 AM
alabamasho kinda threw me a min. Cobalt on that vacuum advance, knew it didn't have that, lol. but I'm leaning toward that whole dizzy being bad. Has had a slight miss for two years I owned, progressively got worse. Now it almost stalls out if i get on it. and have changed plugs and wires, and rechecked individually. Wish fatherinlaw was still around, had big salvage yard here in AL. had plenty of extra parts to try when he was around,, never been puzzled this bad before, something minor just can't put my hands on it yet. Thanks everyone again, I luv this site!!!!!!!!
07-23-2012 10:32 AM
Pre-Tuner Did you change the plugs and wires? I would assume so but you didn't mention it.
07-23-2012 10:08 AM
sbchevfreak This was not a big issue in 91, as the dizzy does not use vacuum advance.
07-23-2012 09:39 AM
Originally Posted by alabamasho View Post
Cobalt guess that'll be next step. Pretuner is that the black bolt-on inside of cap? just changed it, too.
The small black unit held on w/2 screws is the module. That may have been what he was talking about; the pick up coil (below) is the part that is closest to the rotor wheel. The leads from one end of the module go to the p-u coil. Sometimes the two small wires will become brittle from heat and fumes and will break from the movement they get every time the vacuum advance can changes the timing (the round hole in the bracket at 11 o'clock is where the vacuum advance arm fits).

07-23-2012 09:38 AM
sbchevfreak Also, the magnet on the reluctor in the dizzy was known to crack and cause all kinds of headaches, from random misfires, stalls, low power, no starts, ect. I've seen this several times.
07-23-2012 08:58 AM
alabamasho Cobalt guess that'll be next step. Pretuner is that the black bolt-on inside of cap? just changed it, too.
07-23-2012 07:49 AM
Pre-Tuner The pickup coil in the distributor is a common problem with these. They are cheap, so you may want to consider replacing it anyway.
07-23-2012 07:17 AM
Originally Posted by alabamasho View Post
yessir, got it timed to 0. with wire unplugged. feel good about that, still has miss. new tank, pump, sending unit, filter, and new tbi with new injectors. recently installed wires plugs, cap, and rotor. startin to think distibutor itself is bad. got me stumped. rechecked all plug wires and plugs. no CEL on.
Have you put a vacuum gauge on it or checked the compression? You could have a broken valve spring, a burnt valve or a bad cam lobe.
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