My 1990 Bronco with a 5.8 started running REALLY rough so I took it to the dealership to have them do the diagnostic. They told me that the ECM was saying that there was an air/fuel mixture problem. They said that was due to my intake plenum being really dirty and one of the runners was plugged. They wanted almost $900.00 to pull it and clean it so I decided to do it myself. I pulled the plenum and found that it was dirty but nowhere near plugged. What I did find was that the first four ports on the intake manifold were as dirty as you would expect for an intake with 250K, but the last four were considerable cleaner and the very last one (#8 cylinder I think ) was almost clean. What gives? I thought #8 was always the dirtiest because of where the PVC is located. Also if the plenum isn't plugged then what could be causing the air/fuel mixture to be off? Injectors maybe?

 

PVC is down in the lower plenum. The upper gets dirty from the front (nearest the throttle body and EGR valve) to back. Choking is the worst problem especially in the throttle body and in the area just behind in in the upper plenum.

On a more astounding note... they really told you the reason was for a lack of airflow through the intake? Give me the dealership's number so I can call and laugh in the service managers face! (Or tell him what I think of his technician's diagnostics knowledge).

Do yourself a favor and pull codes yourself. When you have the code numbers, post them here.
Thats a cop-out. There are a dozen or more fault codes that basically mean that. The ONLY way to go about rectifying the problem is to know WHICH one(s) are being thrown so you can troubleshoot that particular cause.

 

I checked the codes several times with a code reader and kept getting different codes Which is why I took it to the dealership to begin with. The only consistent code that I found was a coast clutch solenoid circuit failure for the transmission. I didn't have too much faith in these people which is why I decided to pull it apart myself. When I get it back together I will read the codes again and let you know. From what I remember I was getting about 10 or 12 different codes that weren't related. One code I remember for sure was the TPS sensor was bad. I replaced the TPS sensor with a new one from Napa and read the codes again and got the same TPS sensor code. I checked all of the wires that I could find and they all looked to be in good shape. So at this point I'm ready to roll the truck off a cliff!

 

By the way, yeah it was a total cop-out. It was obvious that these guys didn't know their *** from a hole in the ground.

 

Have you checked the obvious things, i.e. plugs and wires all tight and connected. Did this just start all the sudden?

 

I know it sounds like I'm sending you on a wild goose chase here but you might try clearing the memory in the computer and taking the truck for about a 20 minute drive. When you return, run the KOEO one more time and see what codes come up. Remember that CM codes will flash out twice in succession AFTER the engine ID code which should be 4. CM codes should flash out as the same code TWICE followed by the next one TWICE. Once you are out of the CM codes, each KOEO code will flash out once.

What I listed above is the visual you will get from the "Check Engine Light" not what your code reader will show you. I use the "old fashioned" method to take the margin for error out when dealing with a code reader that I may not be familiar with.

 

Yep. However, with my code reader I get the number of cylinders, then KOER shows each code, then shows each code again, then a 10 for separator then CM each code then each CM code again.

May I suggest check all the grounds and battery connections/cables. Next, if you have plastic vacuum lines go ahead and replace them. You will do it eventually so get that out of the way.

While chasing my transmission problem I talked my way past the Ford service writers to the shop forman, a geezer almost as old as me. He had been there awhile and could not remember the last time a Bronco was in the shop. I said mine may have been it with a $360 charge for the $60 clock spring about five years ago. They just don't get to solve our old vehicles problems much anymore. Their techs have retired and the shop manuals they have are not as good as the ones today.

We have said for years the PCM (computer) rarely fails. Mine did and was giving a transmission fault code. After replacing everything, the problem was the PCM. Repaired computer from the speciality shop in Houston gave me a new vehicle. Again, don't jump to fast, but computers do fail.

I trust tune up is good, no wires moved recently causing cross firing?

 

Once I had the plenum off I went through all of the basics, shorted wires, vacuum lines, etc. The plugs and wires have less than a 1000 on them but I checked them all anyway. I replaced the computer about 4 years ago with a remanufactured unit from napa so I hope it's still good. At this point I'm thinking it may be the computer though. I checked the codes twice and had a bunch of different codes with the engine running and with ignition only. I cleared the codes and checked for codes again to make sure they cleared. I then drove about 15 miles, came back and checked the codes again. I got the same result, different codes each time with a few that were consistent, (now that I think about it) coast clutch solenoid, TPS sensor out of range. I think I've eliminated the mechanical stuff, Plugs, wires, cap and rotor, free flowing intake. I'm thinking maybe computer, fuel injectors, electrical short or some silly little $5 sensor. If the computer or injectors are bad how would I now for sure? I don't feel like throwing $800.00 away on injectors if I don't need them. By the way, yes it was all of a sudden that it started running rough. I went out one morning and it started up just like normal but ran REALLY rough. Also I put a new long block in it about 40K miles ago so everything from the heads down should be good.

 

You might want to study this site then call Johnny. They do have test vehicles to try it on. Ford GM ECU ECM PCM TCU Lexus Toyota Honda Acura repair used Mitsubishi

They repaired mine so well the transmission never shifted so good. I am now a raving fan of thiers.

 

"Yep. However, with my code reader I get the number of cylinders, then KOER shows each code, then ....."

Oops! I meant KOEO first...sorry about that.

My PCM issue also hit suddenly but, on the highway.

 

Yeah, b4, the reader I have doesn't show the ID code it just starts with CM. But then mine is a cheapo thing that I don't trust as well as I do my own ability to count.

 

Well I think I may have found the problem. It looks like a mouse or something chewed through the rubber boot that covers the wire bundle that the computer plugs into. There are several wires missing insulation so I'm guessing that there is some kind of short in there. This could explain why I keep getting different codes. I'm going to patch the wires up and see how it runs.

 

Ah critters, don't ya just love 'em? I had an entire family of field mice "move in" to the space between the headliner and the roof of one of my hearses. Ate a hole in a 15' long x 6' wide piece of upholstery fabric that spanned the entire ceiling of a 21' long car. You don't even want to know what it cost to have a new one sewn and installed.

 

On a side note, I was planning on installing edelbrocks intake plenum and manifold setup and also a cam. does anybody have a suggestion on what cam I should use? I already have ford racing headers and a dual 2.25" inlet single 3" flowmaster muffler with no cats. I use the truck for towing my dirtbikes around, exploring logging trails and getting around in the snow mostly. I'm not looking for anything extreme, just something that will give a bit more power and not sacrifice too much MPG.

In stock form the engine felt restricted and it seems minor changes like these will help a lot. There was a noticeable difference in torque after i installed the exhaust so I'm thinking that the free flowing exhaust and a good intake system should make a big difference in HP and TQ. What do you guys think?

 

RV cams tend to be the most common suggestion around here. I've never had a need or desire to change mine but that is the suggestion I have seen most frequently. As for actual item numbers or grind numbers, I can't help you but someone can.