re-occuring problems, can you please help my friend

Hi Folks, a friend of mine is having problems with his 89 5.0L 4x4.

I'll just paste his question

- Truck's running fine, good idle, good power, smooth - when suddenly it's like you've pulled 3 or 4 plug wires off and it starts to stumble, loses almost all power and won't idle at all at a stop light - you have to keep revving the engine while waiting for the light - if you don't the engine just dies out. During these times, you can literally put your foot to the floor and can only manage about 30 - 40 k regardless of which gear you're in. It is more manageable if I can keep the rev's high, by not shifting up, but it still runs extremely poorly. It balks and stumbles - it's like you're popping the clutch and the truck literally lurches (like a beginner driver not letting clutch out smoothly).

Now - the other thing that has been happening when this has occured lately is that the "Check Engine" light has come on and I finally got around to checking the code on it and it indicated "Oxygen sensor excessive lean". This makes some sense to me as it's running as if it's starving for fuel during these times. I changed the oxygen sensor last week and it ran ok for 2 days, then the problem re-occured. I'm assuming there was nothing wrong with the old one - just indicating the lean state in the exhaust, but I changed it anyway just to be sure.

I've also, replaced the fuel filter twice the past 6 months, even cut it open the second time but couldn't see anything in it. I've also replaced the rotor, dist. cap, plugs, oil, etc. and checked timing - all seemed fine.

I'm told it's probably not the fuel pump as it would most likely just stop running period and not start. I've also been told it could be the distributor - pickup problem not allowing injectors to fire properly ? Both places I've taken it to couldn't find anything wrong, but I'm sure it's because on both occasions the problem didn't rear it's ugly head. I'd change the distributor, but it's around $300 for a new one and they won't take it back if it doesn't solve the problem - I just hate to put more money into this heap if it doesn't help. It may be my imagination, but I'd almost say the problem gets worse as the tank gets emptier - maybe excessive water in the tank ??? I know it's a stretch, but I put in 1 L of methyl hydrate this morning and I'll try that for a couple of days with more fuel.

So... I'm at a loss as who to take it to or what to do with it (other than a long ride off a steep cliff...). Anyways, thanks for taking the time to read all this and any help you could give would be greatly appreciated !


Could you guys possibly help him out


Thanks
Overboard

It sounds like a fuel problem..the lean exhaust code is a clue.
When it happens, take a look at the injectors to see if both are spraying properly.
Also since this happens at lower fuel levels, take a fuel sample and see if there is any foriegn material in it.

Thanks GM Merlin. I just relayed the message to hem