• Welcome Visitor! Please take a few seconds and Register for our forum. Even if you don't want to post, you can still 'Like' and react to posts.

Low idle after start, runs fine


CAW

New Member
Joined
Aug 12, 2007
Messages
14
Reaction score
0
Points
0
I have had an extremely difficult to diagnose idle issue that has been ongoing for years. It doesn't happen always but seems to be getting more severe recently. The truck is a 1997 4.0 ohv with 160k miles.

When I start the truck it will start without any problem. But after it starts The rpm will decrease till it reaches slightly less than 500rpm as if it is searching for the lowest possible idle speed. Then the engine will shake like it is about to stall. I can see the lights dim when this happens. On occasion it will stall. But most of the time the rpms jump back up to 1-2k, rather than stalling. If I wait a few seconds longer the rpm will settle at about 700. After that, it will idle and drive with no problems what so ever.

On other occasions, If I put it into reverse immediately after starting(to back out of a parking space), It will shake like it is about to stall for several seconds. I can feel the engine "pulsate" as I apply the accelerator slightly to back out of a parking space. After a few seconds the engine returns to normal.

In either instance the problem goes away if I rev up the engine. The truck does just fine after that. It idles with no problems when I get stuck at a stop light and does not have any lack of power or acceleration problems when driving. The problem seems to occur mainly when the engine is warm, say when starting after a quick trip to the store. It has happened a few times on a cold engine but almost always when warm.

I did a smoke test to check for vacuum leaks. The egr valve leaked and was replaced. The iac valve was leaking smoke and replaced too. The valve cover gaskets, upper and lower intake gaskets were previously replaced due to a lean code p0171 & p0174. So, I'm pretty sure that a vac leak can be ruled out since the poor idle does not happen any other time than right after starting.

I have heard that a bad coolant temp sensor can effect the air fuel ratio calculated by the pcm and cause rough idle issues. I used my code reader to check the engine coolant temperature sensor's live data. All the values were about what I expected; 60*F cold starting and 205*F after running for a while.

Next I checked the fuel pressure regulator. I hooked up the pressure gauge to the fuel manifold and primed the pump. The gauge read 38PSI then slowly dropped to about 30PSI over the next 1/2hr. I was under the impression that the pressure should hold steady for a few hours? I unplugged the vac line to the FPR to check if fuel was leaking past it but found none. This leaves either the fuel pump check valve or leaking fuel injector as the source. Would a leaking injector have these symptoms? I wonder if when I make a short trip to the store, the fuel system is pressurized and leaks while I am inside. Then I start it and have the idle problem. And perhaps when I let it set overnight the fuel that has leaked out has time to evaporate?

I have been trying to chase this problem down for years and am at my wits end. I am reluctant to go and replace all my fuel injectors to test my theory as they are very expensive. Does this make sense or am I way off on my diagnosis? Any help would be greatly appreciated!
 


Rock Auto 5% Discount Code: F9A1A579ACFAD1: October 1st, 2021

RonD

Well-Known Member
Joined
Jun 2, 2012
Messages
20,128
Reaction score
3,951
Points
113
Location
canada
Vehicle Year
1994
Make / Model
Ford
Transmission
Manual
If fuel pressure held at 30psi after dropping it is OK, if it kept dropping then there is a problem.

Next time engine is warmed up and then shut off, "to go into the store", do the "clear flooded engine" routine when you come back.
All fuel injected engines have this routine.

Key ON
Press gas pedal to the floor and hold it down
Try to start engine

If shouldn't start and shouldn't fire even a little
If it fires then yes there is fuel leaking into the intake, probably from an injector since you checked the FPR.

If it just cranks, no start no fire then fuel leaking into intake isn't the problem

Clear flooded engine mode shuts off fuel injector pulses but leaves spark working.
As soon as you release the gas pedal injectors will start working again, even while cranking.

If test shows leaking injector, then after engine is cold, unplug the coil, and repeat the test, then pull out 1 spark plug at a time and check it for fuel, all should be dry except the plug from the cylinder with leaking injector.
 
Last edited:

CAW

New Member
Joined
Aug 12, 2007
Messages
14
Reaction score
0
Points
0
Thanks for the reply!

I just tried the gas pedal to clear the cylinders trick and it didn't fire. I also played around with the fuel pressure gauge some more. Priming the pump takes it up to 40PSI. Engine running takes the pressure to 30PSI even when the engine is revved up. I disconnected the vacuum line to the fpr and it rose to 40PSI again. Looks like my leaky injector idea might not be it after all. Any ideas?
 


Top