2001 GMC Jimmy
4.3 v6 4x4

Sometimes when I go out to my truck, it will start, other times it sounds like it wants to but wont. The weather/humidity don't seem to be a factor. I'm at a loss as to what could be causing this.


What I've replaced so far..

ECM
Spark Plugs
Cap and Rotor
Wires
CPS
TPS
MAFS
Radiator
Fuel Pump
Fuel Regulator
Fuel Spider
Hubs

 

I've also replaced the ICM with a part one I found in the junkyard and still no luck. Every few hours I've been going outside to see if it will start, and the first 3 times I tried, the vehicle started just fine, the 4th time it was a little bit slower to start, the 5th time it didnt want to start but after about 8 seconds or so it finally started, and the 6th time it wouldn't start. 7th time it still wont start. It cranks just fine, but it just wont start. Any help would be greately appreciated.

 

Have you tested the fuel pressure?

How good is the spark intensity?

Have all of your parts been from junkyard motors?

 

yes I've tested fuel pressure, i was assured it was in the proper range. No i haven't checked spark intensity, how do I do that? and lastly no the only part from the junkyard is the ICM, everything else was brand new.

 

What were the pressure readings and how long did the pressure hold after the pump shut off? These motors are very particular about fuel pressure.

You can test spark intensity with a spark tester or by just pulling a plug and grounding it. It can hit pretty hard so I wouldn't recommend holding onto the plug.

 

I don't recall what they readings were. I will be heading back out to the garage within an hour or so. I will recheck the fuel pressure and how long it holds for, along with getting a spark tester and checking that out. As soon as I have those readings I will post asap!

 

What should the range be for what my spark plugs should be able to jump? My friend says that the spark tester is only showing my spark plugs jumping around 25..which I think is low.. or perhaps the spark tester isn't very good... or we aren't using it properly (which I kind of doubt) What can cause a weak spark? I just replaced the wires almost a year ago (to the day) but I've only driven the vehicle maybe 7 months out of that time (I'm constantly fixing things on this jimmy)

Fuel pressure showed the pressure to be 55-60, and it holds indefinitely after the pump shuts off.

I tried starting the vehicle again about 15 minutes ago, and it refused to start, but 3 minutes ago I tried again and it starts... I know its not going to continue to start, I give it half a day max before it stops starting again.

 

nice to see you again swartlkk. i havent seen you in a while

 

i agree with XGIO, welcome back, and as always your help is so greatly appreciated.

Edit: I Replaced the coil with one I found in the junkyard (but once again I wont know if it works until my jimmy wont start)
I'm at such a loss for what can cause this..

 

The spark should be a nice bright blue spark that should jump a 1/4" gap without too much trouble. I'm not sure where those numbers come from, but that is what I have used for years on the modern high energy ignition systems.

Here are some tech articles which may help:
Engine Cranks But Does Not Run
No-Start - Engine Cranks OK - Ignition System Diagnosis

If you have any questions about the diagnostic steps provided let us know.