I have a bone stock '90 iroc 350 tpi, tuned it up plugs,wires, cap,&rotor. Took it down the road and it ran like a striped ape. Next time I drove it, it was missing on acceleration. It doesn't seem to be doing it any other time. All cylinders are firing at idle(noticeable difference when pulling plug wires)and can't hear or feel it at cruising speed. Could something else in the firing system have been week and went bad? If so what can I check and the procedure please. I guess it could be fuel related but from the looks of the tune-up parts that I replaced it was long over due anyways. Any help is much appreciated.

 

I have an '88 IROC TPI 5.7 and had the same happen to me. Once I found out it was a vacuum hose, I simply replaced ALL of them. Good thing too, I found 4 more that we're cracked. Rubber ages & cracks when stored...

 

Did you apply dielectric compound to the plug boots? Make sure that all the plug wire connections are solid and that none of the wires are arcing to ground. As has been suggested, check all vacuum lines for hardness and cracking. You could have a plug that is either broken or defective so that it is grounding out under high KV load. If the ignition system and vacuum lines check out, a bad injector would be the next thing to look for.

Since the condition only takes place under acceleration, idle tests aren't likely to help you find it. If it were a secondary ignition leak, some water sprayed on the wires might point it out. But if it's fuel related, you will need to load it, either power braking or on a dyno, while performing a power balance test. You could just go over the injectors with an ohmmeter and see if any test low resistance(less than 12 ohms is no good).

If the condition is new since the tune up, start by rechecking your work.

 

A spark tester is an invaluable tool. Ensure all of the ignition is tip top before going further. Vacuum leaks don't have as much effect on WOT when there's less vacuum.

 

naf has a good point. Inlet air leaks have more effect at idle where manifold pressure is low (high vacuum). However, on speed density systems, I have seen large vacuum leaks cause throttle tip in issues.

 

10-1 a plug wire went bad, burnt up or came off

 

Ok, checked the wires again couldn't find a problem there, Reran the soft vacume lines with new rubber, the plastic hard lines I left alone they looked good. Still no change.
So what is left in the ignition system to replace.
Where will it be located?
Going to ohm injectors now.
Not saying it doesen't happen but I've chased these plug wires a couple times since install, made sure of the routing and had a defininte change in motor when each were removed from dizzy cap.
It's getting late here and I will have to go to work soon so I'll get back to it tomorrow

 

well...process of elimination...do you have your old stuff? did it work without misfire? if so..start with the wires (i had one only break down under hard acceleration)..then the cap and button ...lastly....plugs..you will find it that way i'm sure

 

Can you replicate the miss by power braking? Sometimes I can, sometimes not. It depends on the cause and how severe the miss is. If you can, then you can use a power balance tester(cylinder shorting tachometer) or any ignition scope that has a power balance function. I do this fairly often on under load misfires to help clarify power balance test results.

It's tricky to do but you can also block the drive wheels and set the park brake, have a helper hold the throttle steady in power brake mode while you drop one cylinder at a time. Try doing the power brake test in the dark while you look for any arcing under the hood. This symptom is reminiscent of a broken plug that I've run into before. You can easily break a plug by side loading it while tightening. The insulator will be cracked right at the base and it may fire well enough at idle but under high KV load(under acceleration), it will leak KV to the metal base of the plug. You can generally see the arc from this in the dark.

 

Ok let me back up here, thanks funkdubie for making me think a little harder on this.
When I done the tune-up the old cap & rotor were pretty corroded and a couple of the wires same as didn't have a boot on them at the spark plug end yet it seemed to have gobs more power than the '03 silverado I had been used to driving around (4,8L dooooog). Anyways 1st drive after tune-up I left the driveway and barked not chirped the new tires in 2ond. Tickled the sheet out of me. this drive only lasted a couple miles and I parked it satisfied with the work I'd done. It was a few days before I drove it again and from the minute I left the driveway I could tell I had a miss. Maybe it's there at idle or cruise speed I'm not sure but defiantly there on acceleration.
This is why I had ask about the rest of the ignition system wondering with the shape of the parts I did replace if maybe something else was week and gave out on or after that first run, if so what might it be and how would I check or test it?

 

Thanks ASE doc, do not have the tool you're talking about. I will check for arc after dark. I've broken spark plugs before so it's possible.

 

We have all broken plugs. I know the high pitched cracking sound all too well. The best defense I have found against it is using a swivel socket where possible and when I have to use a straight socket, I use a swivel ratchet and always try not to side load the plug. We do the best we can.

 

Upon suggestion that I check (ohm) injectors I did. here are my findings,
DS front #1=15 #2-15
DS rear #1=12 #2=15
PS front #1=9 #2=6
PS rear #1=13 #2=14

This was the second time around checking all injectors. Actually the first time around PS front read 5 and 3 respectively. Guess it's time for some injectors huh. Any suggestions?
I had a rather lengthy drive planed tomorrow, could I possibly get stranded?

 

I wouldn't run the motor with those injectors in it. I see burned or tulip'd valves in your future and even wasted pistons/rings. Southbay is the way to go for injectors. They have the Bosch IIIs for cheap. They also give discounts for TGO members.

 

So for reference what should they ohm at? I'm assuming that I need to replace all injectors? and 22lb is stock? Will southbay sell me everything needed to do the job or will I need to get some things elsewhere?

Sorry for all the ?'s and my ignorance, not afraid to tackle unknown jobs but it's been several years since I done much of any mechanical work other than maintenance but I guess if your gonna have a 20+ year old ride you bes get to know it pretty well

 

One more ?, how do I go about getting the tgo discount?

 

The minimum spec for injector resistance is 12 ohms. Southbay will provide you with injectors and o-rings. You will need runner gaskets which you can get at the local parts store. May need to order them ahead. The TPI is a little tricky to disassemble and reassemble. You really want to have a long torx T40 driver to remove and install the fasteners at the base. When assembling, the one rule you must follow is assemble all of the pieces with all fasteners loosely turned in before you tighten anything. The easiest method I've found is to install both runner sets and then install the plenum last. Use vasoline or seal glide on the o-rings.

 

I had an intermittent issue of this sort and I finally traced it to the ignition module. Could be nearly anything, but if its only sometimes, and only under load, it may well be a spark issue, which means it could be nearly anything in the ignition chain. Ignition problems are fun.

 

any help please i have a 1989 2.8 v6 as soon as i press the pedal it does not response it just go low

 

Thanks for the advice. Ordered injectors and gaskets from southbay yesterday. I got a how to post from here saved for reference when I do the job. Just taking things as they come. Maybe this will get it back to running smoothly, if not I'll be back for more testing advice.

 

Juan, You need to start your own thread if you don't find an answer by searching threads. Jumping into a thread like this won't get you the attention you need. A quick answer to your question is check fuel pressure and verify that spark isn't dropping out when you press the throttle. Check TPS and MAP sensors.