Rough Running Dually
 

So I've checked out some threads on here and have followed some advise....but I still need help.
My 91 TBI dually is running rough. It's worse when the weather is cold and especially at start up. Once warm it gets much better. I have not towed in some time but it also seemed to miss much worse when climbing. It would literally jerk from time to time.
Here is what I have done so far
New plugs, wires, cap and rotor
New fuel pumps and fuel filter
New map sensor
New temp sensor

Where should I go next? EGR, injectors, TB base gasket?
Thanks in advance for any time spent trying to help me resolve this issue.

BTW, check engine light is not coming on

Re: Rough Running Dually
 

Check your IAC. Idle air control. Its on the passenger side of your throttle body.

Re: Rough Running Dually
 

Replaced the IAC this morning. No change in the truck. Still runs rough at first start at gets a little better when the truck warms up.

I also tried carb cleaner all around the base gasket and other lines, not jump in idle so I assume that it is ok.

Idle on the truck is right around 800 in park but when first started will jump up and down.

What should I try next? EGR valve? I have not pulled that to clean it to this point.

Re: Rough Running Dually
 

You might. I would simply pull the vacuum line off and plug. Start it and see what happens. Also you should be able to manually operate the EGR to see how s operation is. Also along that line, you can check the EGR control.

Re: Rough Running Dually
 

Ok, I will try that first. I will let you know what happens.

Anything else that could contribute to this rough idle that I should be checking?

Re: Rough Running Dually
 

Start the engine and let it idle. Now, grab the EGR valve with your hand and push up on the bottom to open the valve. The engine should immediately begin to stumble and run rough as it opens and smooth as it closes.

I just went through this on my dually. It wound up being a clogged passage in the intake manifold which I found by doing the test described above.

Re: Rough Running Dually
 

I did a couple of the tests with the EGR valve. Seems to be working properly. I operated it by hand and my truck would run worse with the plunger pressed in and better as the plunger went back to normal.

I am not sure where to go from here. Could it be that my TB needs rebuild?

Re: Rough Running Dually
 

It could. I would also check that; A. IAC is cycling to make your idle go up and down. B. Another thing to check if the IAC is doing its job, is the temp SENSOR. That should be in the intake. Since the sensor is "upstream" from the computer it maybe not operating properly. I'm not sure whether the sensor is a 1 or 2 wire unit. If it were 1 wire I believe just grounding it would make computer adjust to either open or closed loop. A two wire you would need to check with someone that knows them better.

Re: Rough Running Dually
 

when you put your foot on the gas does it stumble less. tps sensor on a tbi will actually open the gas like a choke would on a carb. when they get weak they wont open the gas enough and cause it to stall or stumble. my tbi grand am done this. the injectors themselves sometimes will leak too causing it to dump to much fuel into and stumble. after that the o2 sensor or the vac lines are the only thing else i can think of.

Re: Rough Running Dually
 

Sorry to resurrect this but I am still having issues with my truck. Most recently I replaced the EGR valve and the truck continues with the same symptoms. Runs rough at start up. Only now the truck has started to backfire when it's cold. Any suggestions on where to go? I'm at a loss and feeling I'll never get to the solution.

Re: Rough Running Dually
 

I hope my info will get you fixed up! I had the exact same problems with my 88' 454 TBI dually I recently purchased. It ran really rough on cold start ups when temps were 30-40 or below. It would stumble and pop and backfire through the exhaust pipes.

I followed everyones suggestions on here and and replaced the coolant temp sensor, tbi base gasket (which was leaking) and checked my EGR and still had the problem just as you are.

I noticed my passenger side exhaust manifold was cracked and I thought that was the problem for sure. So I installed a new passenger side manifold and also replaced both donut gaskets on each manifold-tailpipe connection. The next morning it was cold and it started backfiring again!!! I was so frustrated to say the least just as you are. I could still tell I had a little exhaust leak. It sounded like it was coming from the drivers side. So I unbolted the drivers side exhaust manifold to find the exhaust manifold gasket was shot. This was causing cold air to be drawn into the exhaust stream. It also throws off the O2 sensor reading.

After I replaced that gasket and all my exhaust leaks were fixed The problem no longer existed. The truck runs great now and starts great in cold weather as it should......Well as good as a freezing cold 454 wants to start, lol. She grunts and feel like the starter trying to turnover a chunk of lead, lol.

Anyway....my advice is inspect your exhasut system under the hood. I would almost bet money you have at least blown gasket or two or a cracked manifold. I would almost bet money its on the drivers side as well because I had a huge crack in my passenger side manifold and when I replaced it the problem was still present. The reason for that is because the O2 sensor is located on the drivers side tail pipe above the Y pipe connection.

Hope this helps.

Re: Rough Running Dually
 

wow, I had not even thought of that. I am excited to try this fix. Ironically, I was driving the truck the other day and thought "it sounds like i have a small exhaust leak" I could hear a tick under the hood. I never would have linked the two. I'll try it and see if that helps. Much appreciated.

Re: Rough Running Dually
 

After 2 days, a broken exhaust bolt, and a few ruined spark plug wires...I got my gaskets replaced. The manifolds had no visible cracks but I did have a small gasket leak. It's nice to have it fixed but the truck is still back firing when cold.
I am going to re-check timing.
My nephew is a GM tech so I drove the truck in to his dealership. None of the techs could come up with an idea of what caused the backfire or the rough running.
Since my truck has thrown the EGR code, I have to believe it is related to the EGR system.
I appreciate all the help that I have received. I feel like I am out of parts that can be replaced with really no change in the trucks running condition.

Re: Rough Running Dually
 

Does it backfire only on a cold start up? Does it backfire after its warmed up?

Re: Rough Running Dually
 

Not doubting your abilities but after thinking about it make dang sure you have the firing order correct and your wires in the right location. Another thing to remember is to disconnect the computer wire on firewall going to distributer when checking your initial timing. If you check you timing or try to adjust distributed while running the computer will correct it and throw everything off.

When I bought my dually with 454 everyone said that the factory set 454 tbi at 0 degrees on the initial timing (with wire unplugged). Alot of people said that if I wanted it to rum better to advance my initial timing to 4-8 degrees it would wake it up some. I advanced to 5-6 degrees and made a noticeable difference in how she ran. Be careful not to go to far. Of you do it could cause pinging and your knock sensor will pick it up and retard the timing and reverse what you just did.

But double check your firing order and then double check your initial timing

Re: Rough Running Dually
 

As far as I know, only when it is cold. I haven't towed with it in a while or taken any steep inclines but it never does it under normal driving conditionsNo sweat, thanks for the help. Firing order is good. I have double checked and all is well. I will re-check timing next thing and maybe try advancing just a hair. I'll make sure the wire on the firewall as well, I have made that mistake in the past. I'll give it a go and report back.

Any one else bumping up their timing a little?

Re: Rough Running Dually
 

Also check your ignition module. My 88 Crew Cab was bucking under load and it ended up being a failing ignition module.

Re: Rough Running Dually
 

will do. I think I replaced it, but when I pull the cap I should be able to tell.

thanks!

QUOTE=eldogg;6558725]Also check your ignition module. My 88 Crew Cab was bucking under load and it ended up being a failing ignition module.[/QUOTE]

Re: Rough Running Dually
 

My next thing to suggest was the ignition module also. They can be finicky when they go bad. I do believe it is something simple just gotta figure out what.

I also wanted to make sure you double checked all of your vacuum lines. Make sure everything is in its correct place. I think there might be a diagram on your radiator support showing this.

You have said it does it when cold and under a load most of the time. I wanted to ask if it backfires as much if you rev up in park to 3-4k rpm and hold it there? Once your holding the accelerated pedal and rpms steady does it still backfire and rum rough or does it smooth out?

The reason I ask is because when your butterflies are open further it equalizes the vacuum system more (I hope I said that right). So if your butterflies are open then the vacuum isn't being drawn through the gaskets and therefore wouldn't act as a leak. I hope that makes sense.

Also get some starter fluid and some time and spray around your intake gaskets and the TBI.

Re: Rough Running Dually
 

IMO you must start at the very basics for engine problems..

1. Remove all spark plugs, do a compression check and record the numbers
if there is a problem here do not go any further before correcting the problem.(more then 20 psi difference between cylinders shows wear)

2. Go through the entire ignition system, check things you would not normally check, distributor shaft bushing wear, pickup coil , module , coil.

3. Set ignition timing correctly , be aware of the wire that needs to be disconnected for proper initial timing setting. TBI is like no other square body.

4. Check for vacuum leaks at the throttle body base and EGR, TBI engines will run bad with leaks

5. Check the fuel pressure, just because the engine is running doesn't mean it correct. 1 lb of pressure makes a huge difference on these engines


These are all starting points, remember, start with a healthy engine and you will find the problem, do not give up.

Desert