Boe-on

Hi I am a newbie to the forum. Ok Looking for some Chevy Yoda wisdom here. My truck has a 7.4 liter motor with 90 K miles. Been a very reliable truck. It recently developed a symptom similar to a mis-fire. I thought it was a fuel filter issue to start, replaced it, truck ran great (was cool not cold after change out) for a bit then repeated the behavior (after truck warmed up). Towed it to the shop,( He had to warm it up for it to repeat the behavior) Tech said the code was suggesting a random mis-fire. Since the truck had not been tuned up since I have had it (18K miles), he suggested a tune up. after a meticulous replacment of wires, plugs, rotor, rotor cap and air filter it ran great (was cold) then repeated behavior (after it warmed up). I continued driving to get it home when all the sudden it started running fine again. Drove it for about another 25 miles with no problem. Parked it for two hours, came back, started it up and it ran fine, once warmed up it repeated the behavior then after driving it for a while behaving poorlly it cleared up and ran fine. What is going on here? Does anyone have some insight that can help? <!-- / message --><!-- BEGIN TEMPLATE: ad_showthread_firstpost_sig --><!-- END TEMPLATE: ad_showthread_firstpost_sig --><!-- edit note -->

RacerX

I dont know the 454-8 that well but tell me does it use a ignition spark control module? And where is it on this car? On the 350-8 they put it on the right side valve cover. On the Cadillac 300-8 they put it on the intake manifold. That is one of the things that could cause problems when it gets hot and resembles a misfire condition.

You didnt post the year so I dont know if you use a MAP sensor or a MAF sensor. But a bad MAF wouldnt cause that, but a rotten rubber hose to a MAP sensor could simulate a misfire because the PCM wouldnt have a reading from the engine at the very moment the MAP sensor gives no output.

Also, faulty ignition switch, loose battery cables/terminals, PASSKEY issues(if applicable). What else??????

Boe-on

Hi,
So sorry....it's a 2000 year. The Ignition control module is located on the backside of the Plenum, aka upper intake manifold, between the fire wall and Plenum and I learned this nifty terminology from the mechanic.

It has a MAF sensor. Having a MAF, would it also require a MAP? I am a DYI but don't know enough of the terminology to know What all the acronyms mean. What does the acronym PCM represent? I see it in the manual but the manual is pretty general being that it covers 1999-2006 GM trucks.

What is a PASSKEY? is it a security device?

RacerX

do you have the Haynes manual?? They suck donkey ***** on "a Jimmy pulling hogs".

If you have a MAF, then you dont have a MAP. PCM is the engine computer. Bad MAF gives idle and all rich/lean conditions. Bad MAP car no run.

Passkey is the GM Antitheft system. It has a start inhibitor in the ignition switch that will not let the car run if the wrong key is used or will disable the fuel system if the frequency in the key doesnt match the wires in the steering column.

No codes came out of this thing? Random misfire code isnt specific enough to steer you in a direction. All it means is that there was no fuel or spark signal in some cylinder at some point. Did he check vacuum hoses? Made sure no electrical junk was routed past an exhaust pipe or anything? Distributor or Coilpacks?

Boe-on

LOL Nah it's the Chilton but it sucks nuts too. Plus my 40+ year old vision makes the pictures hard to figure out. No MAP and no PASSKEY. He did get a code but I don't recall what it was. I'll need to call him. He said it represented a random mis-fire. SO now I will take your advice and look for that wire that may be melted or a hose that may be deteriorated. Funny how we think the worst and start digging in only to find it was something very simple. I hope I find this to be true for this situation. I'll let you know. Thank you very much for your help to date.

RacerX

random misfire is like P0300. After that they name the exact cylinder the misfire occurs in p0301(cylinder #1), p0303(cyl. #3) and so on.

and your vision is not bad, its the pictures that are. I used Haynes to do a heater core in my car when I had a Grand Marquis. I started in October 2003 and finished in MArch 2004. I was only able to finish the job when I got a better manual with better illustrations from MOTOR...which unfortunately is now Chiltons(as is Haynes).

Boe-on

OK, I Borrowed a friends CarMD scanner and found only to codes when connected, P0300 and P0155. According to my Donkey nut sucking Chilton manual P0300 refers to nothing more specific than a random misfire, and the P0155 refers to "upstream Oxygen Sensor, Right Cylinder bank, Heater Performance". I checked all wiring for shorting and loose connections. I checked all hoses and connections for deterioration. Found the positive battery cable was slightly loose at the battery terminal, tightened it up. I drove the truck and SOB is doing it again. I pulled the right cylinder bank Oxygen Sensor (assuming right cylinder bank refers to the passengers side?). I found a bit of residual coolant inside the harness connector as I do have a slight coolant leak somewhere (another issue for another time I hope) but the coolant was not enough to cover the pin connectors in the harness. Perhaps there is a correlation?

Does anyone know if there is a way to bench test an Oxygen sensor? Am I going down an unecessary trip down a rabbit hole?
Any help will be appreciated.

Boe-on

I found the cuase of the misfire. The Right bank upstream Oxygen sensor harness pins had barely noticable greenish corossion on them. I was able to fashion a swab using a small long handled screwdriver with some lint free swab material taped on it then sprayed that with some brake cleaner and gently cleaned each pin. Then I sprayed the brake cleaner into the harness (not on the sensor itself) to remove any other debris, used compressed air to dry it out and reconnected. I drove the truck for quite a distance and it did not repeat the problem. Now I need to find out how the coolant got onto the harness in the first palce.
Note, the harness is located in a very tight spot behind a heat sheild on the rear passenger side of the block just above and behind the starter motor. I could not find a way to disconnect the harness clips for better access as the wires dive into another larger wire chase which leaves about a 6 inch tail to work with. Pain in the you know what but I figured it out. Thank you so much for your help and I hope this post will help someone else faced with a similar issue.

RacerX

Now you have to locate your coolant leak.

Lester C Chapman Jr

all right fellas. I have a 98 3500 chevy here that does great just as the truck you all were talking about. and it will suddenly just fall on its face. no power. misses out. then it might pick back up and go on again.ive checked fuel pressure checked cats. done tp swap. cleaned egr. now. it had egr code and tp code. went away when I cleaned and relaced. drove truck done good for 3 miles with scanner on it. then bam... did it again. looked at scanner and number 4 cylinder is mis firing into the 100 count. forced it back towards shop. got a block away and it cleared up and ran great. someone tell me what the blanket blank is going on