Howdy guys I just picked up this truck for $500 and trying to fix her up. Its the 5.0 EFI, 2wd and has np-435 trans. So far I've put on a new air filter, capped the egr because the pipe was cracked in half and was causing some issues, put on a new radiator and hoses. It always starts right up idles great but when driving the truck wont go past 3500rpm. Doesn't sound like a rev limiter. If you go WOT when its doing it the intake just gets real loud but doesn't make a difference. It will stutter a little bit when it happens. When i got it the air box was cracked, no filtering was happening just sucking in air so the TB was pretty dirty. That's fixed now but I'm curious if its the exhaust manifolds. Both sides are cracked in multiple places. All thats left of the exhaust is the cats after that its just cut off in the middle (not my doing). While the trucks running there is barely any exhaust coming out the pipe, all of the gasses are going out the manifolds I think. My question is would this affect the O2 sensor causing my issue?? Also let me know of any other things to check. I hate to be another poster that looks like he doesn't search but I have been for hours and haven't found any real solutions.. Thank you for your time!

 

Yes, exhaust leaks will mess with your O2 sensor. Also eliminating the EGR will affect mileage a bit - I know from experience, but it shouldn't affect running.

Check your fuel pressure. Likely you have a failed or failing pump. There are in tank pump(s) and a high pressure frame pump. There's also a filter on the frame. When I lost my tank pump it would run up to around 3k then stumble above that. Get a fuel pressure tester, rig it up so you can read it while you drive and see what happens. I clicked the hood on the first safety latch, not all the way down and ran the hose around and zip tied the gauge to the windshield wiper arm. Only took a couple of minutes to diagnose as all I had to do was drive down the street and rev it up a bit...

 

Great! I actually found my fuel pressure test kit in the garage tonight. Totally didn't think to use it. I have tried driving the truck switched on the front and rear tank and it didn't change anything. So I'm guessing its the high pressure pump?? I'll try to test tomorrow. Thankyou!

 

High pressure pump or clogged filter. Test it to rule it out. Once I hit around 3k pressure started dropping and once it got in the teens it started crapping out. Not sure that's your problem, but rule it out.

 

Sounds good. The next thing on my list to do was the fuel filter and and change all the fluids anyways.

 

Your cats could be clogged

 

That's my next guess after the fuel. Especially with cracked manifolds. Rule out the fuel system then whack the cat off...

 

Same problem on my '96 351 check for vacuum leaks especially on the throttle body and intake

 

Alright I drove around with the fuel pressure gauge on my windshield. While idling it reads 30psi. Then at 3rd gear WOT with the truck not reving past 3200rpm it read 38psi. For some reason it only ran up to 3200 today not 3500... So I'm not seeing a pressure drop but these pressures sound a little low to me. Still need to look up the specs. Also the gauge holds at 35psi after i shut the truck off. Thanks for the other ideas guys!

 

The idle and WOT fuel pressures are low. Idle should be in the 32-35 PSI range. WOT should be 42-45 PSI range.

 

Thanks for the info! I'll try to pick up a filter tomorrow and see if that helps I guess. Could these low pressures be a faulty pressure regulator? I forgot to pull of the vac line to check if fuel was leaking by.

 

Sounds like you have a combination of several problems. The low fuel pressure can cause it to run lean but you would probably start to run rough and possibly backfire at high RPMs if you were running lean.

The cracked exhaust manifolds can cause A LOT of problems. It will absolutely affect the O2 sensor. The computer will think the engine is running lean (due to the excess oxygen introduced through the leaks) and drive the system way too rich. This extra gas will in turn clog up the cat over time leading to a damaged/clogged cat. The cat can then break apart and clog up the muffler, etc. Bad chain reaction of events. Also cracked exhaust manifolds will not dissipate heat as well and possibly lead to burned valves in the engine. The computer should be ignoring the O2 sensor at WOT though.

You can put a vacuum gauge on the engine to check for clogged exhaust. You should see a stable 17-21" at idle and then slightly higher at off idle RPMs. Hold the engine at 3,000 RPMs and the vacuum should remain steady. If you see it slowly drop down as you hold 3,000 RPM then you have an exhaust restriction. If you are not at 17-21" at idle then you have other problems such as a vacuum leak. If you are at high altitude vacuum readings will be lower (about 1" per 1,000 ft). You can also blip the throttle. You should see it drop to about 2" or so then return to about 24" and then back to normal. If it is slow to return back up to normal, that is a good indication of clogged exhaust as well.

 

Great! I'll try and throw the vac gauge on tomorrow and see what that turns up. I'm leaning towards thinking the cat is clogged. I forgot while looking around under the truck the smog pump line going in before the cat was unhooked when I got it.

 

That can be a problem as well. The way the system is designed, it needs the additional air from the smog pump to help the Cat work properly. Without it it may build up deposits over time from excess Hydrocarbons. Anyways, the vacuum gauge test is a good quick test that is easy to hook up and doesn't involve taking anything apart. Let us know how that goes.

 

That's what I've heard. Will be sure to let y'all know tomorrow! Thanks