5 cylinder not firing

[3point8]

#5 cylinder not firing

I have an 85 GN which has been converted to an 87 ECM and an 87 coil and module. Ran fine for two years then this summer I installed a new engine harness, new injector harness, new valve springs, new rocker arm shafts, new 87 style crank sensor. I also installed a set of used green stripe injectors.
Over the summer I put it all back together and it runs rough, missing on #5 cylinder.
Here is a list of things I have done so far (none of which helped)
-changed the spark plug with a new one
-changed the coil and module
-changed the #5 injector to one of the -previous set that worked fine
I also put in a higher stall converter and converted the brakes to vacuum boost but I don't think that should affect anything.
I pulled #5 plug again and it is still wet and black like the first one, I decreased the gap to .025 still no joy.
I pulled the driver's valve cover to insure that the rocker arms were moving and the push rods were moving as they should. Compression on that cylinder is 135 lbs.
I didn't touch the cam sensor BTW.

Any ideas?

Peter Cruickshank

Forgot to mention that I also installed a new fuel filter and a Racetronix hot wire kit for the fuel pump.

 

[jastrckl]

seems like the only thing you haven't checked out is a plug wire. if it's a problem with something like a coil it will miss on a pair of cylinders b/c of the way DIS works. That you're only having a problem with the #5 hole makes me think it's just a wire, since you haven't mentioned shuffling them around. very simple, but sometimes we overlook the simplest things. HTH

 

[3point8]

#5 cylinder not firing

Thanks for the response I shall try that and post the results.
Peter

 

[3point8]

#5 cylinder not firing

Well I tried a new spark plug wire, still no success.
I also swapped in a different ESC module.

The only thing I can think of is to check timing of the spark in relation to the fuel injection and compression stroke.

Anyone know how to force it into limp home mode? Maybe I can just pull the chip?

Peter

 

[turbo6man]

Does it just run rough at an idle and clean up into the higher rpm's. I once had a small block chevy that would drop a cylinder from idle to about 2k rpm. Checked everything from vac leaks to swaping injectors to elcctronic troubleshooting(checked all the things you did). I checked the compression which was fine(within 10% of the other cylinders). After all that It ended up being a broken valve spring. The valve train ran quite, and the valve spring looked fine until I took it off, and it was broke in two. It sure was wierd how compression was fine by cranking it over from the starter, but when you ran it @ idle, that one cylinder would be dead. Best of luck Turbo6man

 

[3point8]

#5 cylinder missing

The compression on the other cylinders ranged from 125 to 130 something.
I actually installed new valve springs from John's Performance. I didn't check for a broken spring just took it for granted they were OK.
The engine misses at all RPM's although somewhat less noticeable at idle. I can't figure out why even if its a valve problem that I don't get partial combustion happening.
Unfortunately I have to do yard work this weekend so it will be next weekend before I can take another look at it.
Thanks for the suggestions though, its looking like a valve problem according to archive search info.
Peter

 

[jastrckl]

what does the plug look like, just out of curiosity? fouled? and yes to get into limp-home mode, you pull the bigger chip, and it will run off of the calpack chip.

 

[Quick6'n'-K.C.]

if #5 is wet, then how does #2 look like?

since 153
426

number 5 and 2 fire off the same coil, then check number 2 to see if its having the same problems.

could be bad plug, could be bad coil pack for #5... you said you swapped module, what about the coil pack?

what did the wire ohm out to?

its probably just something simple

BW

 

[1badTTA]

If it is #5 only and not #2 and it is ignition related, it would have to be the plug wire since a weak spark is all that is required for the "wasted spark" and it will spark when not under pressure but not when on the compression firing. Change that plug and wire and check #2 and that coil. Bet your problem is right in front of you.

 

[3point8]

#5 cylinder missing

OK I shall recap things to answer your queries.
All the plugs are tan brownish except the #5 which is black and wet looking.
I did change both the coil and module at the same time (bought a spare from RedRegalT for just this type of thing)
I didn't check the wire for resistance I just bought a new one from NAPA. I might just try a third new spark plug to rule out coincidences. I want to check the compression again to see if the valve spring idea someone told me about is deteriorating now that I have driven it some.
Shorting out all the other spark plug wires makes engine nearly stall but shorting #5 has no effect. That's using the vacuum hose trick on the coil pack terminals BTW.
Thanks for all the responses guys I really appreciate it. I'm thinking its simple too and there must be an error in my observations somewhere. Yard work weekend is over so I hope to get at it next weekend. I shall report back and let you know what I find.
Peter

 

[QuickWrench]

Given the previous posts on this subject, let's check the opposite side of things...Looking at the pieces that you replaced, 2 catch my eye: The injectors and the injector harness. It's possible that the problem may not be that the plug isn't firing, but that the cylinder is being flooded by the injector. That would also explain why tightening the plug gap to .025" didn't cause the cylinder to fire: that would just make things worse.

So...

Check to see if the injector is actually pulsing. You can do this by picking up a noid light at AutoZone, or using an LED-based 12V tester. Alternatively, you can use a mechanic's stethoscope on each injector to determine if it's actually pulsing. Since the car ran OK before, try swapping back the old injector harness. Although unlikely, they can have shorted leads.

If you don't have one of the testers I've listed, watch the fuel pressure on your guage (which I KNOW you have ) immediately after you shut the car off. Does the pressure drop very quickly, as if an injector is hung open?

Does the car smoke, or spit any raw fuel out of the tailpipe(s)?

 

[wlaukaitis]

nice lobe

HI

Do you have the original cam in the beast? I was wondering if the cam is original with maybe a few 10k miles, that the new valve springs have rounded out a lobe. Not to the extent of completely round but sort of a mole hill versus a mountain. What does it do at WOT?

.02 dollars worth

Bill

 

[3point8]

#5 cylilnder not firing

Hi guys,
Yes I do have an AFPR and a guage. When I installed the hotwire kit the pressure went up over 60 pounds and I quickly adjusted it downward. The gauge doesn't drop after shutdown for at least 20 minutes. Before I installed the AFPR it would hold for over an hour. However you bring up an interesting point. I did notice that the fuel pressure gauge doesn't respond as quickly as it used to at key on. Takes a minute or so to get up to final pressure reading and I did check the electrical connections on it. I must admit that I have wondered about the injector harness also. The reason I haven't done anything about it is because its one of those jobs that takes so long but I'll have to bite the bullet soon. There doesn't appear to be any raw fuel coming out of the exhaust pipes and I don't notice any more odor either.
It is an old cam, but I don't think its original engine was in very clean condition when I bought the car four years ago and the rest of the car was so dirty it would make you sick. So I suspect its been overhauled or replaced with a rebuilt. The reason I dismissed the cam idea is that prior to this project it ran fine and even with the new springs it should last a few minutes running on 6 cylinders. Never tried WOT too worried about it doing damage from vibration.
So now with all your input I'm leaning toward the broken valve spring theory and the injector harness.
Coworker thinks I should have replaced the rings, honed the cylinders and checked the bottom end, cam etc. while the engine was out anyhow.
Maybe I will do that anyhow as I notice the lifters are noisy too.
I was somewhat depressed with this car but with all the help I'm getting from you people I decided I'm going to lick this thing even if I have to take the bus to work for awhile.
I'll keep you posted on progress and thanks again for your input.
Peter

 

[3point8]

Update

Today I bought a NOID light kit as Quickwrench suggested and discovered that #5 injector harness plug doesn't light up the lamp at all. I checked the new lamp just in case it was faulty but it lit up when checked. So its looking like that new injector wiring harness needs a closer inspection.
I didn't throw out the old injector harness and it worked fine so maybe this next weekend I'll get to work on it.
Still wonder how come the spark plug is black and wet looking if the injector doesn't fire at all??
Peter

 

[QuickWrench]

Still wonder how come the spark plug is black and wet looking if the injector doesn't fire at all??

It's just black from the oil and fuel inside the cylinder.

After you replace the injector harness, you can test it before firing the engine by unplugging the ECM ground disconnect (passnger-side, by the battery). One by one, crank the engine with the noid light plugged into each injector's harness clip. If they all check out OK, replace your #5 plug, hook everything back up, and you should be back in business.

 

[Jimn8or]

Check the pins where the injector harness meets the wiring harness for any bent or otherwise kafukulated pins. Next, ensure that none of the injector wiring for #5 is pinched by a zip-tie or valve cover/doghouse [a grounded injector wire has been known to keep the bugger open and flood the plug/cyl.] Next be certain that the O-rings are perfect. Even the smallest nick will create way too much fuel if it's a lower one and way too much air if it's an upper O-ring. Did you swap the lead with another from the engine or another one all together? Change it out with one that is proven already on the engine.
HTH
Jim

 

[turbosam6]

I bet its the injector harness, which, btw, takes all of 10 minutes to change, but check this: I noticed you put on a new coil/ module. One time I did that and pinched one of my injector wires under it, and cranked it down good. It didn't miss, but I did notice it later. Its worth a look. Good luck.

 

[3point8]

Finally firing on all six

Well after much tearing of hair and gnashing of teeth I got to the bottom of my problem.
I left the #5 injector connector off after checking it with the NOID light last night and immediately got a code 43 malfunction. Didn't think much of it at the time but after I got back to it tonight after work I traced the wiring from injector back to the ECM. No connection! Just a few minutes ago I found it. The yellow wire in B12 that fires #5 injector was the wrong wire. The yellow/blk stripe wire in B7 was connected to the injector. So I swapped the two and it fires on all six now. B7 s/b wired to the ESC module. Whew! Never suspected a wiring error in a new wiring harness.
Thanks for listening guys and all your input I appreciate the support it was pretty depressing there for awhile.
Peter