Hey guys I have a strange question about my truck running rough. The truck is a 1992 5.0L automatic. Since I bought the truck I have done some work done but the truck has always ran a big rough or odd to me. I always chalked it up to its an older truck but the other day someone mentioned they heard it missing? It seems to idle around 800 rpms and seems to shake a little bit. Its not horribly bad but it is noticable if your not use to driving an older vehicale. It almost feels like it wants to stall when sitting at a light like the idle is dropping a bit. I know I wont get a 2012 running motor I just want it to run top notch.

I have no codes stored, the plugs were replaced before I started really driving it, not even 3000 miles on them yet. Any thing I should be looking for? The truck does run good But with gas prices I want to make sure it runs at the best it can.

 

Is it possible it's running on 7 cylinders? I'd start by opening the hood at night and watching the engine run, you might find a broken plug wire or another electrical problem.

 

I dont think it is running on 7 cylinders. Ill try looking it at night. Ive had it running a few weeks ago in the dark and didnt see anything, wasnt looking for it either but didnt see anything jump out that caused me to be concerned. Once you start driving it runs good and has no problems. I was thinking maybe a fuel filter because I havnt changed it yet but I would think it would do it all the time no mater what

 

Running rough and/or no code misfires are one of the harder things to pin down.
Several problems can cause them, and you can have more than one problem.
The best way I have found it to be systematic in the approach. I pay attention to what order things are done and what changes occur or don't occur because of what I have done.
The more detail we have, the better we will be able to help you. A couple of questions come to mind, how many miles are on your truck? Do you have access to tools, particularly testing tools? Also, if I understand you correctly, nothing has recently changed about how the engine runs?

A quick vacuum test will give you a general idea of the mechanical condition of your engine, a compression test is better. It takes a bit of time to do it correctly, but if you have one or more cylinders with compression issues you are not going to be able to get it to run smoothly no matter what you do. This is more likely on a high mileage engine.

I wouldn't replace anything other than basic tune up items without testing them first. It costs money and given the quality of parts these days may actually make things worse.

Some things to think about:

Have the air and/or fuel filter been replaced recently?
Did the rotor get replaced with the plugs?
How about the pcv valve?
Can you hear any hissing under the hood from a possible vacuum leak? I listen carefully from a few different angles, they can be hard to hear (sometimes impossible). If you aren't sure what to listen for, pull a small vacuum line off the intake and cover most of the hole with your finger.
What color is the inside of the tail pipe? Should be light gray, roughly the color of a cigarette ash, darker means its running rich, lighter means lean. this is assuming you aren't burning oil or taking in coolant.
Does it use oil or coolant?
Does the engine start right away, or does it crank a bit first?
Does it rev up smoothly or hesitate?
Does it smooth out when rev'd up, or still run rough?
Does it smoke out the exhaust when started? When rev'd up? Going down the road?

Having a look at your spark plugs is a good place to start doing actual work, I ould check compression at the same time myself. Make sure to note which plug came from which cylinder paying particular attention to any that look different than the others and also the general condition of all of them.

 

I have acess to tools. The milage is 165000. When I start it it would crank then fire up. It is better know that I have a new starter in there. As for some of your questions it does not seem to burn oil or lose coolant that I have seen.

Air filter is new and has around 3000 miles on it

I have not changed yet the fuel filter, plug wires, rotor,and same with the PCV valve

No vaccum leak that I can hear. I had an issue with a coffe can having
holes in it but I fixed that.

The tail pipe color looks fine

The rev seems smooth,hesitates here and there but seems to be maybe transmission related, converter shudder

When accelerating it smooths out and only seems to be rough at idle

I dont see any smoke out the exhaust except the normal cold morning starts.

 

Did you gap the plugs when you put them in? Its not unheard of for them to get dropped on the floor in a parts store and get the gap closed. I've had 2 friends bring me their vehicles because they were missing after they changed the plugs and that's what it was. I also suggest this because you said you changed the plugs before you really started driving it so perhaps it was fine beforehand? Otherwise a compression test would be wise. Then make sure you're getting spark. Then make sure you're air filter is decent. Then I suppose fuel would be next. Change the filter regardless if you haven't done it yet. Check pressure and make sure your injectors are good. If all those things check out it should run like a champ. Oh, be sure to check your timing too.

 

The plugs were changed while me and my mechnic were doing some work. Air filter is good as it is a new drop in filter. Ill put a new fuel filter in it and pcv valve. It might be my timing chain, I do remember my mechinic while working on it saying it sounds like my timing is lose, Maybe thats why I think its running rough. If I put a new chain on it might pick up the idle to smooth out a bit more? Also a side question about the fuel filter. Can I remove it with out draining my tank?

 

Plug wire routing:

courtesy of Steve83


Very common problem on these trucks: cross-fire caused by improper spark plug wire routing.

Absolutely no reason to drain your fuel tank(s) to change the filter. Relieve the fuel pressure by depressing the schrader valve on the fuel rail. Or pull the fuel pump fuse/relay or trip the inertia switch with the engine running, wait for the engine to stop. Then change the filter. Be aware, you will get some fuel spillage due to fuel left in the lines and old filter. Do not be alarmed...

 

So the wires not being routed right will cause an issue? I dont mean the wire from the distrbutor to the plugs. I mean like wire 3 has to go under wires 1,4,2? I double checked that from the distrbutor to the plugs that they are right. Wire 1 from distrbutor goes to plug one ect

 

You need to route the wires as shown in the photo. Keep wires 1 & 3 separated like the photo shows. Same goes for 5 & 6 which is more critical than 1 & 3 because one cylinder fires right after the other.

 

Oh ok. Ill check that. I just noticed it seems to drop idle from around 800 or so to 500 when it seems like its running the worse. I would think that what ever is doing that is my issue. But Im going to check the wire routing

 

Start with the easy things first.

 

I re wired 5 and 6 they were next to each other. I have to pull my wires apart on the passanger side, there zipped tied so they won't hit my headers. Anyway started it up after changing my 5 and 6 wires and its up to around 1200 Rpms. What is the normal for idle?

 

Normal is around 800 as far as I know. 1200 seems quite high.

 

This is the same thing that is happening to my truck