98 4.0 with bad idle, stalls,rpm decrease when accelerating

[Rosscoinc]

Hello,
I have a 1998 Ranger 4.0 XLT 4x4 and I am completely baffled by the following:
What I have done or what is newly installed -
New crate engine with 10,000 miles
fuel pump
fuel filter x 2
MAF 2 years old, just cleaned
all new relays
new coolant sensor
new throttle sensor
new K&N filter
tune up (plugs, wires) at engine install
visual inspection for vacuum leaks. 1 found and it runs worse when plugged.

What is happening:
Just before leaving for hunting the truck stalled out and would not start seeming like it was getting no fuel. It would start after pumping furiously on the throttle for a minute or two. I replaced the fuel pump and filter and it ran fine for 2 days then started doing the same thing. Very hard to start whether cold or hot. When I am pressing on the accelerator it will do fine but when it idles it will go up to 2000 rpm (roughly) then down then up, down, up down etc. When it finally settles down it goes to 500 rpm or so and goes up and down a little then it will eventually stall. I cleaned the MAF and then it got REALLY bad. After changing all the relays it seems a little better except that now when pressing on the throttle it dogs out and almost stalls. I have to gentle pump the throttle to get any rpm's. It does start ok now after it warms a little bit but seems like no fuel when first starting cold.
I am at a loss at this point. Do I spend the money on a new MAF? Get a vacuum system test? Anything else to try? Please help as I do not know what to try next. Coil pack maybe?
Thanks,
Ross.

 

[bmerr98]

Are you disconnecting the battery when changing these sensors? If not maybe leave it disconnected for 5-10 minutes and see what happens. Just starting with the basics...

 

[swynx]

I was gonna say 02 sensors. Unplug them see if it still happens. Next thing that came to mind was the fuel pressure regulator. But its a new engine that should be good.

 

[pcollins]

Idle Air Control, commonly referred to as the IAC. Check that out and clean/replace it. Also, what did you use/how did you clean the MAF? The tiny little wire inside that gets hot is very brittle, easy to damage. Might explain why it got worse after cleaning.

 

[Rosscoinc]

Did not disconnect battery when changing relays. Will disconnect for 5-10 as suggested and see what happens.

The fuel pressure regulator was used from the old engine. May swap that next depending on cost (running out of dough on this beast...).

Will check/replace the Idle Air Control tonight.

I used Lucas MAF cleaner in the spray can being careful not to hit that wire too hard with the spray.

Thanks for the replys. This is killing me!!! I need my truck!!!

 

[Rosscoinc]

O2 sensors were all replaced with engine swap but I guess thats no guarantee they are still good. Will check that as well.

 

[Rosscoinc]

PS - the Fuel Pressure Regulator was replaced with a new one at engine swap but as with anything else I guess it could be bad.
Tonight I am going to pressure test the fuel line.
How do I vacuum test? I visually inspected but I suspect there is a leak somewhere. I have heard of a smoke test but do not know if I can do that (how?).
I replaced the 4x4 vacuum engagement with locking hubs a couple of years ago so I dont think that is the issue but...

 

[bmerr98]

Disconnecting the battery will allow the computer to reset and start fresh with the input from your cleaned/replaced sensors. A little something I've learned here.

 

[bigmark303]

You may have easily burnt the MAF sensor wire if you did not let the wires dry completely before starting the engine again. The wire gets hot and will burn if not dry of the cleaner while engine is running.

Also you mentioned truck would not start and you had to depress the gas pedal multiple times and it eventually started. That leads me to believe the engine is flooding because when you depress the gas pedal while turning over it cuts off fuel flow. If the truck started after you depressed the pedal and cranked it had fuel.

What is the engine throwing for codes? $50 buys you an OBD reader and its much less expensive than throwing parts at the truck while guessing.

 

[feellnfroggy]

was the vacuum leak on your vacuum tree? maybe you have a hose disconnected?