Random Misfire

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Brentbba

Brentbba

Former Golfer
10 Year 15 Year 20 Year
Joined
Mar 27, 2003
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448
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11,074
Location
OC, CA
Random Misfire - Help Needed.

Little background - HG replaced 2.19.10 with the newer HG.

Head was slightly warped and they send it out for milling. Don't know how much was removed, but was told it was pressure checked at the machine shop - no cracks.

Engine has what the shop is calling a 'random' cylinder misfire that they can't figure out. It only started after the HG. I'm the only owner and it's never done this. New plugs with the HG and they even threw in another set yesterday, they double checked the valve adjustment yesterday also, compression checked each cylinder, checked for air/fuel mixture, said no vacuum leaks. They've done a dye and leakdown and all is fine. Timing is dead on as well per the shop.

Anything I can let the shop know to check? Sometimes the truck idles very smoothly. There's just the occasional 'stumble' (and it's getting more frequent as time goes on) that they've referred to as the random misfire - not the same cylinder per the shop. Acceleration is very smooth. Misfire, stumbe, whatever you want to call it is noticable at idle in park and worse at idle, fully stopped in drive. I make sure I'm at a complete stop to feel this with KM2's so I'm not mixing the tire rumble with the engine!

Shop suggested I try to find a someplae with an old cylinder scope to see if it could detect what was going on.

Ignition wires, Dist cap & rotor were all replaced in August of last year at a Yota dealer and the fuel system cleaned.

Talked to the shop again this afternoon and their manager had done some add'l research last night (he's baffled too) and asked how many miles on the truck - 203,000 and it's a '94. He said 'maybe' the distributor is going bad? Ignition coil?

Any thoughts, comments, suggestions.
 
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how was the wiring harness condition near the egr tube that tends to deteriorate from heat and would have been moved around by the head re and re during the hg work and when the injectors were reconnected? i'd suspect that first, especially if the shop that did the work did not know to be careful with those wires and check them.
 
does the misfire disappear when you are in drive but not moving?

that would make me suspect something in the wiring to the iac, tps or maybe the injectors.
 
Mine has had a misfire at idle since I bought it. It bothers me but not enough to spend the $$ to diagnose it. I've been meaning to find a coil to swap out but I don't want to waste the money on a new one if the old one isn't bad. And I don't trust using a used one from the junkyard. Mine was same way before HG change and is still same after. And will probably be the same for a while longer. I wish i could be more help.
 
Oh and I replaced plugs(bosch iridium), wires(toyota), cap(toyota), button(toyota) during the HG install.
 
Mine does the exact same thing... It started after a plugs, wires, cap, and rotor tune-up (all OEM parts).
I think its caused by one of the following:
1. A crack in the intake tube
2. A faulty plug wire
3. A compromised engine wiring harness

Best of luck to you! I'll be trying to fix mine asap...
 
desmocruiser said:
Check the wiring harness. Mine had he same misfire, replaced the engine wiring harness and it disappeared. Mine was compromised in a couple of spots.

https://forum.ih8mud.com/80-series-tech/72268-strange-idle-problem.html
Click to expand...

Everything you described in your thread is EXACTLY what's happening. Only difference is that I don't have an S/C.

I'll check my wiring harness too. Out of curiosity - what's an engine wiring harness run? At 16 years of age, I'm sure it's brittle and worn all over the place.

Will also, to the best of my :banana::wrench: ability, test the distributor and coil.
 
brent, it is prolly a wire fault right near the egr tube. this is a known problem due to lack of heat shielding and does not indicate the whole harness is bad. it can be fixed with a wire splice. i would not replace the whole harness, but i would get that section of the wiring harness wrapped in heat insulation and carefully placed.
 
Mine was wrapped in heat insulation by PO. I need to open it up and inspect it for damage.
Do the wires just get brittle and fail?
Are the wire failures obvious and visible or do they require testing?
What are the wires controlling exactly? Ignition / timing, fuel, O2, ECM in general...?
 
darnold87 said:
Mine was wrapped in heat insulation by PO. I need to open it up and inspect it for damage.
Do the wires just get brittle and fail?
Are the wire failures obvious and visible or do they require testing?
What are the wires controlling exactly? Ignition / timing, fuel, O2, ECM in general...?
Click to expand...

don't bother testing. the simplest thing to do is to just splice in repairs to all the brittle sections of the wires and see if it helps. it was only a few inches of the harness that needed attention and it will take less time than running tests. i used butt connectors and heat shrink for the splices then wrapped it in hear insulation from napa.
 
IIRC the harness is around $385.00. It's a bear to replace but I've done a few now. My thought is that splicing at the problem area is a temp fix since when your done the bundle is not only larger but won't flex as easy either. IMO it's the physical contact with the EGR as well as the flexing that creates the problem. And the patch seems like it would be more of a problem.
 
semlin said:
brent, it is prolly a wire fault right near the egr tube. this is a known problem due to lack of heat shielding and does not indicate the whole harness is bad. it can be fixed with a wire splice. i would not replace the whole harness, but i would get that section of the wiring harness wrapped in heat insulation and carefully placed.
Click to expand...

And where specifically is that? Remember - I'll still only call myself a :banana::wrench: .
 
landtank said:
IIRC the harness is around $385.00. It's a bear to replace but I've done a few now. My thought is that splicing at the problem area is a temp fix since when your done the bundle is not only larger but won't flex as easy either. IMO it's the physical contact with the EGR as well as the flexing that creates the problem. And the patch seems like it would be more of a problem.
Click to expand...

true but 60k later my patch is working just fine.
 
semlin said:
how was the wiring harness condition near the egr tube that tends to deteriorate from heat and would have been moved around by the head re and re during the hg work and when the injectors were reconnected? i'd suspect that first, especially if the shop that did the work did not know to be careful with those wires and check them.
Click to expand...

this

semlin said:
true but 60k later my patch is working just fine.
Click to expand...

Mine's been patched twice now. Probably should have just replaced it the first time, but that was over five years ago, iirc, and not many people had experienced the problem yet.
 
Ok - who can point me to where the suspect section of the wiring harness is? Pics would be a great help to me on this one. Is it near the EGR Pipe as shown in the FSM? Way back at the back top of the engine? '94 FSM page EG-189
 
brent i don't have a photo but look just behind the throttle body towards the back wall of the engine bay and you will see a small plastic flying saucer shaped doohickey with a green or red label on top and three black hoses coming out of it. this is the egr modulator. follow two of those black line further back toward the engine bay wall and they will connect to a cast iron valve body which is your egr valve. climb up onto the fender and look down over the egr valve and you will see the egr tube that enters from the bottom. just beside and to the right of the egr valve and pipe is the section of wiring harness that goes bad tucked down out of the way. there are numerous wires there so it is about 1/2" thick . it will probably have the remnants of the factory protective coating which may crumble in your hands.
 
semlin said:
brent i don't have a photo but look just behind the throttle body towards the back wall of the engine bay and you will see a small plastic flying saucer shaped doohickey with a green or red label on top and three black hoses coming out of it. this is the egr modulator. follow two of those black line further back toward the engine bay wall and they will connect to a cast iron valve body which is your egr valve. climb up onto the fender and look down over the egr valve and you will see the egr tube that enters from the bottom. just beside and to the right of the egr valve and pipe is the section of wiring harness that goes bad tucked down out of the way. there are numerous wires there so it is about 1/2" thick . it will probably have the remnants of the factory protective coating which may crumble in your hands.
Click to expand...

Ok - and how do you access it to splice it? Perhaps a good inch thick? Runs back from the throttle body area up past the EGR pipe to the top of the firewall and then takes a sharp left over toward the PS of the engine bay and down and in to the ECU behind the glove box. From what I can see it looks in ok shape from the outside. A little heat shielding on the very top but not where it sits on the EGR pipe?
 
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