Rough running and backfire through intake only when warm

[5.0limited]

This is on my 98 explorer 5.0 i thought it was running hot replace thermostat clutch fan water pump now the cooling system doesn’t seem to be getting hot and it just runs really rough and a pop through my throttle body once the engine gets up to running temp after I let it sit for a lil bit it cranks up and runs like a sewing machine no weird noises then about 15 min starts doing it again any ideas

 

[rasouth]

A couple of things to try are a compression test to see if you have a leak, spray starting fluid around intake manifold to see if it speeds up, check all the little (and big) vacuum lines, pull plugs to see if any are real black with buildup.
If after running engine for a while you say that the radiator does not get hot or are you saying it gets warm but not hot? I have a '96 4.0 and the temp gauge hardly ever gets above a third of the way up, and I live in SoCal. What is the temp rating on the thermostat? 180 or 195?

 

[5.0limited]

Yes the radiator doesn’t seem to be getting overly hot and my temp gauge doesn’t read anything over normal running temp when I replaced the thermostat they gave me one that was the factory specs though about just taking it out.. problem seems to only occur about 5 min after it’s up to normal running temp my cel has been coming and going hopefully it’ll come back on soon and I can pull what code it’s saying... I’ll try to spray around see if I get a rise in rpm other than a vacuum leak would a clogged cat make it back fire through the throttle body? I just picked up a laser temp I was going to check it too. someone told me my camshaft position sensor could be failing as it warms up causing it to mis fire...

 

[410Fortune]

a backfire through the intake is almost always a lean condition

pull the code
check fuel pressure

 

[5.0limited]

a backfire through the intake is almost always a lean condition

pull the code
check fuel pressure

Thanks good to know I’m gonna try to spray starter fluid around the intake any way to check fuel pressure without any special tools or do I need to maby go try to rent something

 

[410Fortune]

you need a fuel pressure gauge
There is a shrader valve on the fuel rail, just to the pass side of the upper intake manifold
Most auto parts stores will rent the tool
1998 5.0 should have a single fuel line, if this is true then you should have 58-65 psi at the rail
since your truck starts to lose power when its warm I suggest that is the best time to test. It is quite possible your fuel pump is getting hot and then losing the ability to make adequate pressure
let us know if your engine has two fuel lines, in 98 was the switch from one line to two
The return style fuel system (two fuel lines) you should have 36-42 psi at the rail

5.0 intake manifolds are not known to leak nearly as much as the plastic v6 intakes do
Lean condition on a 5.0 v8 is usually fuel pressure related
The computer code is very very helpful, 98% of the time if you have a check engine light then the code will tell you/us exactly where to look. Write down the ACTUAL code number like P0174 do not just take the auto parts store clerks word for it

 

[5.0limited]

This is really weird I spreyed everything down last night didn’t see any increase in rpm cranked it today to try and get my check engine light to come back on hasn’t starting running rough or backfireing let it idle in my driveway for a while took it down the road been running it for over an hour still no cel hasn’t done what it’s been doing for two weeks while I was replacing the cooling system parts the only difference today is here in Florida the temp dropped from the 70s to the 40s since yesterday I’m still go to go rent a fuel pressure gauge and check that just really weird it does appear that it has two fuel lines

 

[410Fortune]

okay then 34-42 psi at the rail is what I like to see
Fuel pumps get hot and when they do they start to lose pressure
The fuel on top of the pump is what cools it
So the ambient temp outside has everything to do with that as well as the amount of fuel in your tank

The other thing it can be aside from low fuel PSI and an intake leak would be the IAT sensor.....air temp sensor in the intake tube on your truck with the 5 wire MAS
(98+ 7 wire MAS the IAT is built in)

 

[5.0limited]

okay then 34-42 psi at the rail is what I like to see
Fuel pumps get hot and when they do they start to lose pressure
The fuel on top of the pump is what cools it
So the ambient temp outside has everything to do with that as well as the amount of fuel in your tank

The other thing it can be aside from low fuel PSI and an intake leak would be the IAT sensor.....air temp sensor in the intake tube on your truck with the 5 wire MAS
(98+ 7 wire MAS the IAT is built in)

Cool deal I have a lil above a quarter tank I’m gonna pick up the pressure gauge tonight is it ok to drive when the pressure gauge is hooked up? guess I could check the pressure cold then drive around in the block a bunch then check again if the psi is dropping probably take a long time gonna be down in the 20s tonight.. figured I could just tie strap it on to something in the engine bay

Also I’ll look to see if I have a 7 or 5 mas I’ll spray it down see if it’s leaking thankyou again for all the help I’m learning a lot

 

[5.0limited]

Update fuel pressure gauge I got the adapter is broke can get any kind of a pressure reading I’m gonna take it back and get another one try it tonight finally got my cel to come back on it’s a p0153 bank 2 02 sensor slow response

 

[5.0limited]

okay then 34-42 psi at the rail is what I like to see
Fuel pumps get hot and when they do they start to lose pressure
The fuel on top of the pump is what cools it
So the ambient temp outside has everything to do with that as well as the amount of fuel in your tank

The other thing it can be aside from low fuel PSI and an intake leak would be the IAT sensor.....air temp sensor in the intake tube on your truck with the 5 wire MAS
(98+ 7 wire MAS the IAT is built in)

Yes you were right! Finally got a gauge that works came up to 30 psi held for a min dropped to 15 then to 20 after it warmed up started running rough almost dropped to zero I’ve isolated my problem to the fuel system thank god lol now should I go fuel pressure regulator or fuel pump what do you think is the best way to test either one of those components?

 

[410Fortune]

I have been around the 4.0 engines and these trucks just a little bit (20 years)
30 psi is toooooooooooooooooo low
Replace the pump and strainer, also the fuel filter

This is the pump I like to run the bosch "blue top"
More Information for BOSCH 69128

you will need a new strainer:
More Information for BOSCH 68018

and a filter:
More Information for MOTORCRAFT FG872

Rockauto discount code good until Jan 12 2020:
123947974113250287

You can easily test the fuel pressure regulator, pull the vacuum line to it and see if it has any fuel in it
If not then the FPR is probably good
When you pull the vacuum line off the FPR with engine running fuel PSI should rise, but with a weak pump this test may not work
The fuel pressure regulators do not go bad very often

You only need to replace the sending unit in the tank if your gauge is not working, otherwise just replace the pump and strainer, ALWAYS with a new filter

good luck! and nice work diagnosing your own issue