I have a problem with my truck in sig, the 5.8l is missing and bucking heavily when first started, at idle and driving down the road. The idle will drop and it will almost die then catch itself.

For fuel pressure I have 32 psi at idle and 38 without the fpr vacuum line hooked up.

I found the throttle body butterflies to be so gummed up the return spring could not shut them under their own strength so I removed the tb and cleaned it thoroughly. Idle set screw not touched, iac valve is a year old, took off tps and cleaned the gum out of it. I tried to test the tps sensor but could not get a good enough reading while trying to back probe the wire pigtail.

After cleaning out the throttle body, the truck still has the issue. After the truck is warmed up it is not as serious but still there, I noticed after driving around for 20 minutes and coming back home putting the truck in park it idles around 900 rpm now when before it was idling around 650-700 rpm.

I do not have vacuum leaks, and by grounding the spark plug to the frame it appears I have healthy spark, I also tuned up the ignition system about a month ago. Timing at 10 degrees.

anyone have ideas? I thought it might be tps sensor or maybe map sensor?

CEL bulb doesnt work so I cannot retrieve codes by counting flashes.

 

any ideas? I need to go pick up a trailer but I'm stuck now.

 

Fuel pressure seems fine...Mine runs at around 29 PSI at idle. Rev it up and goes up to 44 psi...Key on engine off around 37PSI...Going by the BBK fuel gauge on my fuel rail.

Sounds like a TPS issue but it could be other issues. When my 5.0's TPS went bad it was doing that.

The problem now you are having with higher idle is because the throttle body is either clean and sucking more air around the blades and through the 2 holes.

The previous owner of the 5.8L that I got now. Cleaned the throttle body and worn the protective service down inside the throttle body. I thought it was the IAC.... I had to use Silicone on the two holes that go through the blades to limit the amount of air flow. I'm just debating on swapping to a aftermarket throttle body or holding off and just converting over to a single throttle body design for future mods. To test this just put your fingers on the really small holes that go through the throttle flap blades in your throttle body.

You need to test that TPS....and check for codes.....

 

Hey Toplait, thanks for the response.

Now that you bring up the TB butterflies holes, I can remember plugging them with sheet metal screws on my 460 because of the same issue.

I hope it is a simple TPS sensor, I will work on back probing the pigtail again today. When I removed the TPS while cleaning the TB it had a ton of grease/grime and dirt inside it, I tried to clean it out with carb cleaner on a q tip but I think the dirt could be inside of the potentiometer and messing with voltage signals.

 

Ok, installed new tps, old ones voltage range was .56v-2.3v

truck still has the missing and bucking problem.

tested coolant temp sensor, iat sensor both checked out, o2 sensor is also ok.

unplugged iac valve while truck was running and it continued to run the same, which is supposed to happen I think?

anyone else have ideas? this is my only vehicle.

I'm wondering if the ecu is fried

 

Have you checked for codes? Yes, I know it's cliche but we have to ask. Pry open the wallet and spring for a real code reader or replace that $.39 bulb,

The idle RPM should drop to the 400-500 RPM range with the IAC unplugged.

Plug wires,cap, rotor age? Did you inspect for signs of corrosion under the cap?

Inspect the plugs to check for signs of gas fouling/rich condition?

 

Good morning rla2005, I have not checked for codes yet because of the bulb.

The idle did not change at all when I would unplug the IAC and reconnect the pigtail, it just continued to idle very rough around 750 rpm and surge up and down, every so often the trucks idle will fall to around 300rpm or so and then catch itself and surge up to 1000rpm.

cap, rotor and motorcraft copper plugs have roughly 1500 miles on them and are 2 months old. Plug wires are a few years old. I did check over these parts except for fuel fouling the plugs.

also last night I went out and started the truck so I could see if anything was arcing under the hood and there was no arcing.

 

Sure sign of a vacuum leak, IMHO...

Pull the plugs and inspect. If the plug wires were cheapies I would also seriously consider replacing them too. Cheap wires tend to deteriorate quickly. They are also more susceptible to cross-fire. Checked your wire routing.

 

I agree it definitely seems the symptoms of a vacuum leak I just don't know where else to look, I have replaced all lines and checked the intakes over.

I will work more on it today and see what I find.

Thanks

 

Do not forget about vacuum reservoirs, and power brake booster. I had a slight high idle when I first purchased my old '92 F350 w/5.8L engine. No amount of carb cleaner spray or manually testing each vacuum circuit revealed the true cause; gasket leak between the upper and lower intake.

 

I'm currently working on trying to figure out a similar problem on my 1994 with 5.8l. On mine if I give it just a little throttle it chokes and nearly stalls. I ended up changing the fuel filter and pressure regulator and that brought my PSI back up towards 35 PSI running, and 40 KOEO. When messing with it yesterday, I noticed that if I disconnect the vacuum line from the EGR valve and plug it, the issue goes away. In my case, I think it may be related to the EGR system. Maybe try temporarily disconnecting the EGR and capping the vacuum line to see if it helps you narrow it down?

 

Checked every vacuum circuit, including plastic reservoir and booster. Found egr valve is open at idle, I removed it (the rest of my smog equipment was gone when I purchased truck anyway) the egr and made a block off plate out of aluminum and formed a gasket then installed on intake. Also I was not sure of what to do with evp so I removed it off the top of egr valve and plugged it back into the harness and let it hang.

Truck seemed to run much better, idled for 10 minutes and did not almost stall so I took it for a ride, about 2 miles down the road at 65mph it starts doing its bucking and acting like it's going to cut out and die.

Also the truck has not had a high idle, it's been around 700-750 rpm

I hooked up vacuum gauge to manifold tee, and got between 16-17 psi? Or hg? At idle (NOTE THIS WAS BEFORE FIXING VACUUM LEAK AT EGR) forgot to hook gauge back up after fixing the unmetered air leak.

Pulled plugs and they still look like they're new, no evidence of flooding or leaking injector.

What do you guys think? I'm wondering if it could now be an ICM or pip sensor in distributor?

 

But idle RPM should drop or kill the engine with the IAC unplugged. You have (or had in light of the EGR issue) a vacuum leak or someone jacked with the throttle stop screw.

 

With all the problems, my curiosity would have been pulling the PCM out and looking for signs of problems.

You can either keep tinkering with everything else or pull the PCM and inspect. Better now than later so your not chasing your tail.

 

I agree, idle does not change when iac is unplugged. Only place left for vacuum leak would be upper or lower intake gasket.

I cannot confirm if idle stop screw has been touched, I personally have not messed with it.

I am curious about the computer, I guess I'll throw that on the to do list