Stumble during start

[cthudak]

Hi guys,
I have an issue with my 97 Ranger 4.0 . When I start the truck when is cold, it starts fine. When it gets up to temp and I shut down, and start again after 5-10 minutes, there is a slight stumble during starting. The truck does start and runs fine, and no check eng light. Anybody have an idea what might be causing this?

Thanks for the help and the great website

 

[RonD]

Have you noticed a drop in MPG?

Could be a leaky injector or Fuel pressure regulator.
Cold engine can run richer and won't mind, and any fuel in the intake would have evaporated, O2 sensors are not online for the first 5 minutes as engine warms up.
Warm engine can't start well rich, and O2 sensors should compensate for small leak once started, so engine would run fine, just lower MPG

Fuel pressure gauge would ID this, '97 should be 35-40psi running, and not drop below 28psi with engine/key off, check it after 10 minutes of key off.
You could also pull off the air plenum(big air tube) on the intake, warmed up engine, open the throttle and smell for gas, there should be no gas smell.
When you turn off a fuel injected engine, the injectors shut down with the key, then engine stops rotating a few seconds after that, so there would be no gas/fuel in the intake it would have been sucked out by engine rotating.


Could be Coil Pack is starting to go, once it heats up it has a weaker spark, which will still fire running engine, but after shut down, while still warm it takes a few on/off triggers to get/build enough spark.

Starter motor drawing too many amp warm, do you notice it cranks slower warm?

IAC(idle air control) valve sticking closed.
On cranking/startup the IAC valve should open all the way, so RPMs should go up to 1,500, then if engine is cold RPMs will drop to about 1,100
If warm RPMs should drop to about 750.
If IAC valve isn't opening all the way on cranking, then warm engine would have Rich mix so slow to start.


That's all my guess's for now, lol.

 

[rangerraider1979]

I think I have this same problem in my 1994 I start mine it runs fine and then shut it off and then I go to start it again it sputters and after I hit the gas it sputters and spits untill higher rpm's and then it runs fine. So I am thinking fuel pressure also but I know the plugs and wires have not been changed for a long while for my father in law owned it and had not changed it and he got it from my wifes late grandfather who took real good care of it since he bought it in 1994 with only 2 miles on it and he put 1 mile test driving it so its been well maintained we dont know when he changed them last . So I am going to clean the plugs wires look clean. So we will see. Any other ideas.

 

[rangerraider1979]

What about clogged cat or o2 sensor.

 

[cthudak]

thanks for all the possible issues. I'll check each one at a time.