High idle
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What idle RPM does the truck go down too? Does it almost die? If the latter happens that means the base idle screw is properly set and that also points to a bad sensor "telling" the PCM to drive the idle higher. If the idle is still in the 700+ or higher range that most likely means you have unmetered air, aka vacuum leak. Least likely is a bad ECT or MAP sensor.
I noted before, you may be passing the KOEO tests, but that does not mean all the sensors are 100% correct. It just means they are within the normal range the PCM is looking for.
You have had multiple TPS sensors and I think IACs with the same result. It's time to start actually measuring something with a meter. Ryan has some great info here: Fuel Injection Technical Library » Home
Invest in a cheap DVM, hopefully one that can also measure frequency. It will pay for itself in short order. Harbor Freight has several meters under $20 USD. Even one with a frequency range to measure your MAP sensor.
I noted before, you may be passing the KOEO tests, but that does not mean all the sensors are 100% correct. It just means they are within the normal range the PCM is looking for.
You have had multiple TPS sensors and I think IACs with the same result. It's time to start actually measuring something with a meter. Ryan has some great info here: Fuel Injection Technical Library » Home
Invest in a cheap DVM, hopefully one that can also measure frequency. It will pay for itself in short order. Harbor Freight has several meters under $20 USD. Even one with a frequency range to measure your MAP sensor.
2. I have a digital volt meter, but it's at home. When I test the MAP sensor (if the screw is aligned correctly), what should be ideal?
Fleet Owner
Joined: Oct 2005
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From: Kentucky
Everything points to a vacuum leak to me, but just to be sure you need to check the sensors with a meter before tearing the engine down further IMHO.
A MAP sensor will have an output, in hertz, as noted here from Ryan: Fuel Injection Technical Library » Manifold Absolute Pressure Sensor (MAP)
There is no adjustment, if the sensor falls outside that range with a known input signal then replace it.
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You have been through multiple throttlebodies at this point with the high idle remaining. I seriously doubt each one of them has had the throttle stop screw mis-adjusted. Just in case, the screw should be adjusted so there is just a hint of light around the blades when they are pulled against the stop screw. This typically equates to an idle speed in the 400-500 RPM range, perhaps a little lower.
Everything points to a vacuum leak to me, but just to be sure you need to check the sensors with a meter before tearing the engine down further IMHO.
A MAP sensor will have an output, in hertz, as noted here from Ryan: Fuel Injection Technical Library » Manifold Absolute Pressure Sensor (MAP)
There is no adjustment, if the sensor falls outside that range with a known input signal then replace it.
Everything points to a vacuum leak to me, but just to be sure you need to check the sensors with a meter before tearing the engine down further IMHO.
A MAP sensor will have an output, in hertz, as noted here from Ryan: Fuel Injection Technical Library » Manifold Absolute Pressure Sensor (MAP)
There is no adjustment, if the sensor falls outside that range with a known input signal then replace it.
Fleet Owner
Joined: Oct 2005
Posts: 20,789
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From: Kentucky
Lower intake to cylinder head gaskets replaced? Lower to upper intake gasket, too? How about the EGR gasket? You have replaced the IAC and the throttlebody a few times I assume you replaced the gasket each time as well?
If you have cruise control there is a vacuum reservoir behind the driver side headlight. The servo is also vacuum controlled. The HVAC system has a small reservoir located on the side of the blower motor case. The HVAC servos are also vacuum controlled.
By unplugging each circuit one at a time or successively you should have eliminated those before. That is why you are at the point of verifying sensors. I seriously doubt the PCM is causing the high idle, it is almost completely eliminated by unplugging the IAC. There is still some small interaction going on that is why I suggest you verify the sensor outputs are correct.
If you have cruise control there is a vacuum reservoir behind the driver side headlight. The servo is also vacuum controlled. The HVAC system has a small reservoir located on the side of the blower motor case. The HVAC servos are also vacuum controlled.
By unplugging each circuit one at a time or successively you should have eliminated those before. That is why you are at the point of verifying sensors. I seriously doubt the PCM is causing the high idle, it is almost completely eliminated by unplugging the IAC. There is still some small interaction going on that is why I suggest you verify the sensor outputs are correct.
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Lower intake to cylinder head gaskets replaced? Lower to upper intake gasket, too? How about the EGR gasket? You have replaced the IAC and the throttlebody a few times I assume you replaced the gasket each time as well?
If you have cruise control there is a vacuum reservoir behind the driver side headlight. The servo is also vacuum controlled. The HVAC system has a small reservoir located on the side of the blower motor case. The HVAC servos are also vacuum controlled.
By unplugging each circuit one at a time or successively you should have eliminated those before. That is why you are at the point of verifying sensors. I seriously doubt the PCM is causing the high idle, it is almost completely eliminated by unplugging the IAC. There is still some small interaction going on that is why I suggest you verify the sensor outputs are correct.
If you have cruise control there is a vacuum reservoir behind the driver side headlight. The servo is also vacuum controlled. The HVAC system has a small reservoir located on the side of the blower motor case. The HVAC servos are also vacuum controlled.
By unplugging each circuit one at a time or successively you should have eliminated those before. That is why you are at the point of verifying sensors. I seriously doubt the PCM is causing the high idle, it is almost completely eliminated by unplugging the IAC. There is still some small interaction going on that is why I suggest you verify the sensor outputs are correct.
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If the sensors are good, I will start with replacing the IAC valve. I may make a trip to the junkyard and just pick one up. Since we eliminated the throttle body as the cause, I will put the old one back on so I have all the hookups for the fuel vapor box. However, when I set it down, I got a big clot of dirt into the coolant line. I gotta clean it out with one of my pick set tools
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Merry Christmas everyone. Hope you had a good one and are getting ready to bring in 2013. So to resurrect this forum: I finally got around to trying to test the map sensor. I do not have a check engine light coming on. I have a Craftsman digital multimeter. I unplugged the map sensor but since I've never used a multimeter, I did not know where to put the diodes and what setting. Fuel Injection Technical Library » Manifold Absolute Pressure Sensor (MAP) was not any help in that area. It shows what frequencies to look for but am unaware how to interpret the graph and set up the multimeter. Any ideas?
I also looked for remaining vacuum leaks. All my lines are good. The only place I could think of would be the coffee can canister under the washer fluid/ coolant reservoir may be rusted out. I tried to reach down there but I'll have to unscrew it to check. That's all the vacuum lines I can think of.
I am pretty certain it is a sensor now. So far I have cleaned and replaced the IAC, throttle body (I had to plug the vapor box lines as I have nowhere to plug them in), new TPS sensor, all new gaskets, checked all vacuum lines
I also looked for remaining vacuum leaks. All my lines are good. The only place I could think of would be the coffee can canister under the washer fluid/ coolant reservoir may be rusted out. I tried to reach down there but I'll have to unscrew it to check. That's all the vacuum lines I can think of.
I am pretty certain it is a sensor now. So far I have cleaned and replaced the IAC, throttle body (I had to plug the vapor box lines as I have nowhere to plug them in), new TPS sensor, all new gaskets, checked all vacuum lines
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I may have fixed it......
Yesterday I threw on a new map sensor making a little difference. It lowered the idle to 700 rpm after it idled about 1 to 2 minutes (normal operating temp). I also found a rusted out vacuum canister which I will replace. I temporarily plugged the line. I could not find anymore vacuum leaks otherwise. When cold it idles at 1200 rpm. After I give it gas the rpm goes back to 1000 upon releasing the gas
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As soon as I get a new canister, I will search for further vacuum leaks. Since they truck never ran right; should the idle go to 700-800 when I stop the truck, or only when I first start it or idle a while. I.E. on my Durango, when I am driving and not pressing the gas, the truck idles a 1000 rpm when compared to 700 rpm in gear when stopped. I don't know know if I'm hunting a non-issue
Fleet Owner
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From: Kentucky
Cold idle RPM is typically in the 1200 RPM range then it should drop down from there. My 1992 F350 w/5.8L engine would warm idle in neutral between 900-1000 RPM. In Drive it would hold steady around 700 RPM.
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I will see my results with the new canister but I may have fixed it finally. When I have been driving it and depress the clutch (like pulling up to lights), the idle will go up to 1200 then fall to around 7-800 rpm after 15 or so seconds
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I know this is not my thread but if the truck idles back down when the IAC is unplugged that means the ECU is telling it to open up and speed up idle, correct? I testing the FPR on my truck last summer and it held vacuum so I don't think it was the issue in my case, also if the diaphragm is damaged it will leak fuel into through the vacuum hose causing a very rich condition all the time.