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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!
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!
