351W Wont Idle - Diesel Forum

lone_star_dsl · 09-09-2008, 01:12 AM

I posted this a while ago but couldnt resurrect it in a search.

Anyway, my plow truck is a 1994 F-250 with a fuel injected 351. I had problems with it not idling last winter and, unfortunately, I havent touched it all summer.

Sometimes I could plow for hours before the engine would finally idle. Sometimes it wouldnt idle at all. When driving down the highway, it felt as though another cylinder would fire up and I could instantly feel more power and the truck would idle fine after that.

I went out the other day and changed all the fluids and fired the truck up. I was pretty surprised at how quickly it started, but even at 70* outside, the engine wouldnt idle. It would fall off to roughly 200-300 rpms, hang on for a few seconds then quit.

Being a fuel injected engine, I dont know where to start looking for a problem.

Help me out guys, snow season is almost here!

**action4478** · 09-09-2008, 08:53 AM

An easy test would be to use a timing light on each spark plug wire ,one at a time ,,see if they are firing ...

LMJD · 09-09-2008, 09:39 AM

Don't know if this applies to your particular year and engine, but Ford had a TSB regarding the rough idle problem which covered many of their gas engines. The throttle body butterfly plate cruds up with carbon, etc, due to the fact the PCV valve hose dumps crankase fumes in upstream of it. It hangs open enough to screw up your idle air/fuel mixture when air is supposed to be going through the idle air bypass passage. The fix was to pull the rubber intake ducting and spray an aerosol carb cleaner type product on the butterfly valve to dissolve all the carbon.

eagleeye · 09-09-2008, 09:42 AM

I owned an identical truck to include the plow for eight years. Start by replacing the distributor cap, rotor, plugs and plug wires. Spend the extra money and buy good plug wires or you'll be replacing them again shortly. If that doesn't work, make sure your injectors are still seated. I hit a frozen snow bank once and when the truck landed it dislodged one of my injectors, took me 2 weeks to figure it out as the injector was located under the intake. Also make sure your air filter is clean and that your exhaust system is functioning properly, i.e. catalytic converter not plugged. If none of these improvements fix your problem, you might have a burnt valve. You'll have to do a compressiom test to find out.

MarkEkberg · 09-09-2008, 10:35 AM

On these engines it could be a few things.

1. major vacume leak
2. bad ips (idle position sensor)
3. bad maf (mass air flow) if it has one
4. bad o2 sensor. It might have two.
5. major exhaust leak pre o2 sensor.
6. coil could be weak.

Best thing to do is read codes if it is acting up. It will tell you wher the computer is not getting the correct info from. It could be the sensor or something that is forcing the bad info. If you pay the 50 bucks to have them read will save mucho time banging your head against the wall and throwing money at parts