94 E 350 with a 5.8. about 1/2 throttle or under quick acceleration it bucks violently and will die if you don't let up on the throttle. This all started when it was run out of fuel. I found the condenser wire was shorted to its own body. Causing the PCM fuse to blow. I have since replaced the condenser. Same problem exists. I have replaced the map sensor with a known good one, same with the O2 sensor. The TPS is in range and goes through its range smoothly (this was done on my scanner) Also replaced the fuel pump either due to the fuse blowing or running it out of fuel it wasn't working. Fuel pressure is 30 to 32 psi. I've checked the EGR for leaks and proper operation. I have noticed its very hard to start when its cold. Someone has had problems with it in the past as cat is new and i have noticed some things have been removed or replaced recently. The van was just purchased a few weeks ago by my buddy. Any ideas or help would be greatly appreciated.

 

My first instinct tells me the exhaust is severely clogged, obtain a hand held infrared thermometer and start taking readings of the exhaust system starting at the front of the pre-cat while someone holds the engine speed way above idle, the temperature should remain consistent at every point in the system, if the temp drops noticeably after 1 of the cats or after the muffler then you found the problem.
There are lots of people that will tell you the stock exhaust is fine but i have already proven it to be more than restrictive, the stock exhaust absolutely kills engine power..

 

I got this van (it belongs to a buddy and it broke down in my town 200 miles from his house,he just bought it ) when i got in to it i realized it had previous problems. that said, the cat had already been replaced. It has since gotten colder outside and the van is almost impossible to start cold first thing in the morning. I rechecked the tps reading yesterday and realized that it is only reading the voltage not the actual range on the tps. I also checked the temp unit to be sure it was reading the air temp correct and it is. I'm getting to be at a loss as what to check. Once it's warm it starts ok but the drive ability issue is still there.

 

The TPS has a 5V reference signal, when the throttle is closed the signal voltage should be less than 1V but as you open the throttle the signal voltage should increase in a linear fashion until it reaches the full 5V reference signal, if it don't then you found at least part of the problem..

 

yes it reads .85 closed and rises smoothly to 5.0 at ft

 

check and/or replace fuel filter, the pressure is good as you have checked the rail but that was with engine off and definitely not under a load.
the pump could still be bad also, need to check fuel pressure while driving and see if it drops under a load.
I am still thinking....

 

I have since checked the fuel pressure while running and making the van fall out. The pressure is good at the time it goes flat and stumbles. i did find that the coolant temp and intake air temp both read about 10 degrees lower that the actual air temp at cold start up. Also the last time it quit /stumbled on me it was so violent that the engine dieseled and ran backward throwing the belt. After re installing the belt i went to start the engine and found i had nothing. It wouldn't crank and the warning lights on the dash didn't even come on. Neither did the fuel pump pressure up. after turning the key off and on several times it finally came back to power and would start and run.

 

The PCM can't be shorted out (fuse) to many times before it gives up for good, I would check the timing chain and see if it has jumped a tooth or two............

 

I thought of that and didn't say anything because i doubt it wouldn't even start and run if that was the case, the base timing reference would be off a minimum of 6 degrees and more likely 12 degrees or more... but, just for good measure he can pop the distributor cap and rotate the crank back and forth while somebody else observes the rotor button and see how much slack is in it..

 

The pcm fuse is a maxi 30 amp it would pop so hard/loud you could hear it. to find my short i wired in a 30 amp circuit breaker. i now have a 20 amp mini fuse in place. I'm going to put the 30 amp maxi back in today. I seriously doubt that that has anything to do with it ,but going to get it back to original anyway. I used the circuit breaker because it was way cheaper than buying $6.00 maxi fuses by the groves. Also i had a lot of 20 amp mini's when i was done repairing the short. So i used the 20 amp mini as a test fuse. Just never re installed the 30 maxi. Going to check timing today. If timing isn't a factor. I may have to just throw in the towel. I have never been one who just starts throwing parts at a problem. Plus the fact the guy just bought the van and paid ($2800) which was too much to begin with. Makes it hard to start throwing good money after bad. Thanks for all the help.

 

Sorry Dave, you got one that we can't fix and that is sad, i wish we could be be more helpful and get your van running smooth but the issue eludes our thoughts, it could be a problem with the ECM but now we are just guessing, don't throw good money after bad..

 

I'm going to check the timing anyway. But my thoughts are that after the bad backfire and dieseling/running backwards that if the chain was that bad it would have slipped so far out it wouldn't start. Do you think i'm correct in my thought on that? Plus the fact that after that happened it took like 15 mins to get power back to things so that it would crank and fire again. It was the most violent backfire and dieseling i have ever seen.

 

I'm convinced the chain is tight and has not slipped but now you got me thinking about the fuel again... Do that engine have a MAF sensor or is that Speed Density EFI ?

 

timing is dead on and chain is tight. Just talked to a buddy and he said fuel pressure might be way to low. He seems to think it has the booster pump on the frame rail and runs high pressure. Like somewhere in the 100 psi range. I've looked around and cant find the pressure reading for a booster pump equipped vehicle at all. Did find another electrical problem but it's fixed now too. Oh and to answer your question gman it has a MAP sensor.

 

There is no booster pump on a gas truck of this vintage but you should be seeing 39-45psi at the motor with the engine idling and vacuum removed from the regulator, and 32-35psi with vacuum attached. Also pressure should increase towards the upper levels any time the throttle is cracked or the engine put under sustained load(low vacuum). If fuel pressure ever drops below the 32-35psi range at any time you have a weak in-tank pump or a clogged filter.