Engine/Transmission (1994 - 1998) Lower Boost Than Normal

[Sandifer]

Ok, I just rolled 200k miles last week and yesterday I was coming home from work and gave the gogo pedal a good whomp and realized I could't get the boost to exceed 30psi. Usually I can hammer on it in 5th and hit 35psi with no problem. EGT's are no higher than normal. I do think me lift pump might be getting lazy. I have a tick that sounds like a lift pump to me. Could that have anything to do with boost? It starts with no problem. Runs smooth in all gears. I have a 4gsk and today I noticed I couldn't get it to do much over 80mph floored with about 28psi of boost. Any ideas?

 

[RJPotts]

The only reason that boost levels will go down, aside from turbo problems, which I doubt this is, would be a lack of fuel.

If your EGTs aren't any higher than normal, then you probably don't have an air leak somewhere, and it is exhaust heat that spins the turbine, so I'd say that you're not getting the fuel you once had. LP may be the culpret, I'd look there first.

-R. J.

 

[Sandifer]

I checked all my hoses and clamps and all looks good. I drove it to work today which is about 20 miles and the problem is still there. Can't get no more than 26-27psi of boost. Got to work and popped the hood and noticed the ticking noise is pretty loud. Sounds like lift pump. It's not the valves. I could raise the rpm's a little and could make the tick change noise level and sometimes go away. What ya'll think?

 

[TXDSLSHOP]

I think you are on the right track. Might check fuel pressure, could be a weak overflow valve or the lift pump as R. J. suggested. Just my 2 cents. Jeremy.

 

[hsmith]

Ticking noise is probably the lift pump. With high mileage, might change the OFV along with the pump. A little grinding with a die grinder above the OFV will give more room for removal.

 

[Sandifer]

Changed my lift pump today and I still have the ticking noise. Went and drove and my boost is even worse. Couldn't get up much past 20psi. What else could be causing my low boost pressure?

 

[Joe G.]

These things are very seldom the lift pump. If you don't have a fuel pressure gauge then you don't know for sure if the problem is low fuel pressure. The next thing to check just in case you do have low fuel pressure is the overflow valve. The original valve on your '95 is one you can take apart. Remove it and check the spring. It may be broken or short. If it's not broken it should be about 1/2" long or a tiny bit longer. If it is short then stretch the spring a little. Be careful because the spring can be damaged if it's stretched too much. The spec is 0. 550". If the spring is broken then you have to get a new valve. If that is not the problem then make sure the pre-filter screen is not clogged. Have the hoses been changed to go diesel rated hose. You may have an air leak in one of them.

 

[Sandifer]

I have the original overflow on the truck. If the spring is too short and not to spec will that cause low power and boost? Will it cause the ticking?

 

[Joe G.]

It the spring is too short you will have low fuel pressure. If it's low enough then you will have low power and low boost. I can't reply to the ticking since I don't hear well.

 

[Sandifer]

Tomorrow I am going to change my fuel/water filter. I thing it might need to be changed. When I pulled the screen in the fuel heater it was filthy. It was full of stuff that looked like chewing tobacco. I cleaned it best I could before reinstalling it. Could the fuel filter be dirty enough to cause low boost? Could the filter cause the lift pump to tick? It seems wierd that once I replaced the lift pump I ended up with worse performance than before. The boost went from 28psi max to 20psi max.

 

[Joe G.]

Did you check the overflow valve?



You probably disturbed whatever is wrong when you replaced the lift pump. It's best to find what is wrong before replacing parts. Saves money and usually takes less time.

 

[Sandifer]

Now,for the rest of the story. I changed the fuel filter and checked my overflow valve spring. The spring was about 3/16ths of an inch too short so I stretched it out to right at a half an inch. Went and drove truck. Runs great. Looks like I got some bad fuel. Thanks guys for the help.