clay1937
Registered Member
I have a full-size 1991 GMC Jimmy with a 5.7 litre (350 cid) V8, that has recently developed a low idle and searching / bouncing idle problem.
When the engine is cold and is started, it runs fine, with the cold idle around 1,100 RPM. As the engine warms up (I typically let the engine warm up to about 100 degrees before driving), the idle drops down to about 800 RPM.
While driving, as the engine continues to warm up to operating temperature, the idle continues to drop to where it finally reaches it’s normal running 650 RPM, as it has done for the last 14 years I’ve been driving the truck.
The problem that is now occurring starts when the engine finally reaches it’s operating temperature (i.e., around 180 degrees), which typically occurs 2-4 miles into driving, where the idle will drop too low, to around 500 to 550 RPM, and begin to "search" for the right idle, bouncing up 50-100 RPM, then down 50-100 RPM over and over, acting as if the engine is going to stall and die (it actually has died a few times when the weather was really hot).
Once the problem starts, it is only seen when slowing from about 30 MPH to a stop, and when sitting at a stop.
The idle searching up and down is most obvious when the transmission is in Park or Neutral with higher (i.e., 100 RPM) bounces (I usually put the transmission in Neutral at red lights). The idle searching up and down is less dramatic when the transmission is in gear, although the RPM is still too low (i.e., 500 - 550 RPM). Even when it’s not so dramatic, it causes the oil pressure to bounce up and down with the RPM adjustments...the oil pressure is REALLY erratic when the problem is more dramatic.
The problem goes away when pressing or holding the accelerator, which includes taking off from a stop, and just normal driving...it runs beautifully when pressing the gas...there don’t seem to be any ignition/timing problems at all.
Once the engine is at it's hottest, the idle searching also occurs during deceleration from about 30mph down to zero.
The carburetor has been cleaned many times, and the engine has had a Motorvac service.
So far I have already replaced the following parts with no improvement:
Tune-Up Parts (GM cap, GM rotor, AC Delco plugs, GM wires)
PCV Valve
ECM (computer chip)
Distributor Module
IAC and gasket (this is the only part that is no longer available from GM)
TPS (Throttle Position Sensor)
EGR Valve and gasket
MAP Sensor
Knock Sensor
TBI Injectors
I recently read in a repair manual that if the temperature sensor is bad (not the temperature sender), that nothing in the injection system will work properly, so the next thing I'm going to try is replacing the Temperature Sensor, once it comes in (the 4x4 version of the GM part was only available on special order). I also have a GM O2 sensor to try after that.
I've had trouble finding the correct EGR solenoid, but have thought that may be a possibility too.
Other thoughts I've had include replacing the distributor, or even the thermostat (I'm running a 180 rather than the factory 190, but I've been running a 180 for the last 14 years I've owned the truck). I have even considered that replacing the A/C/Heater temperature controls might help, (although this was replaced about 5 years ago when it quit working).
There don't seem to be any vacuum leaks anywhere, and the intake manifold gasket was replaced about 5 years ago, and isn't leaking again yet.
Even though the truck has 209,000 miles on it, I can't imagine that there's a problem with the fuel pump (i.e., low pressure) which was replaced about 8-10 years ago with an OEM GM pump, coil, or timing chain (i.e., play in chain), because the truck runs perfectly until the engine is hot.
And since it’s a 1991 model year, there's no MAF sensor, Crank/Cam Position sensor, or even an air pump. It’s just TBI not SFI.
Someone made a posting of the exact same problem at http://www.automotiveforums.com/t183013.html but that was 4 years ago, and the solution was never posted. User GMMerlin posted a good article that will be helpful to me, but any other thoughts will be greatly appreciated.
Thanks!
Clay
When the engine is cold and is started, it runs fine, with the cold idle around 1,100 RPM. As the engine warms up (I typically let the engine warm up to about 100 degrees before driving), the idle drops down to about 800 RPM.
While driving, as the engine continues to warm up to operating temperature, the idle continues to drop to where it finally reaches it’s normal running 650 RPM, as it has done for the last 14 years I’ve been driving the truck.
The problem that is now occurring starts when the engine finally reaches it’s operating temperature (i.e., around 180 degrees), which typically occurs 2-4 miles into driving, where the idle will drop too low, to around 500 to 550 RPM, and begin to "search" for the right idle, bouncing up 50-100 RPM, then down 50-100 RPM over and over, acting as if the engine is going to stall and die (it actually has died a few times when the weather was really hot).
Once the problem starts, it is only seen when slowing from about 30 MPH to a stop, and when sitting at a stop.
The idle searching up and down is most obvious when the transmission is in Park or Neutral with higher (i.e., 100 RPM) bounces (I usually put the transmission in Neutral at red lights). The idle searching up and down is less dramatic when the transmission is in gear, although the RPM is still too low (i.e., 500 - 550 RPM). Even when it’s not so dramatic, it causes the oil pressure to bounce up and down with the RPM adjustments...the oil pressure is REALLY erratic when the problem is more dramatic.
The problem goes away when pressing or holding the accelerator, which includes taking off from a stop, and just normal driving...it runs beautifully when pressing the gas...there don’t seem to be any ignition/timing problems at all.
Once the engine is at it's hottest, the idle searching also occurs during deceleration from about 30mph down to zero.
The carburetor has been cleaned many times, and the engine has had a Motorvac service.
So far I have already replaced the following parts with no improvement:
Tune-Up Parts (GM cap, GM rotor, AC Delco plugs, GM wires)
PCV Valve
ECM (computer chip)
Distributor Module
IAC and gasket (this is the only part that is no longer available from GM)
TPS (Throttle Position Sensor)
EGR Valve and gasket
MAP Sensor
Knock Sensor
TBI Injectors
I recently read in a repair manual that if the temperature sensor is bad (not the temperature sender), that nothing in the injection system will work properly, so the next thing I'm going to try is replacing the Temperature Sensor, once it comes in (the 4x4 version of the GM part was only available on special order). I also have a GM O2 sensor to try after that.
I've had trouble finding the correct EGR solenoid, but have thought that may be a possibility too.
Other thoughts I've had include replacing the distributor, or even the thermostat (I'm running a 180 rather than the factory 190, but I've been running a 180 for the last 14 years I've owned the truck). I have even considered that replacing the A/C/Heater temperature controls might help, (although this was replaced about 5 years ago when it quit working).
There don't seem to be any vacuum leaks anywhere, and the intake manifold gasket was replaced about 5 years ago, and isn't leaking again yet.
Even though the truck has 209,000 miles on it, I can't imagine that there's a problem with the fuel pump (i.e., low pressure) which was replaced about 8-10 years ago with an OEM GM pump, coil, or timing chain (i.e., play in chain), because the truck runs perfectly until the engine is hot.
And since it’s a 1991 model year, there's no MAF sensor, Crank/Cam Position sensor, or even an air pump. It’s just TBI not SFI.
Someone made a posting of the exact same problem at http://www.automotiveforums.com/t183013.html but that was 4 years ago, and the solution was never posted. User GMMerlin posted a good article that will be helpful to me, but any other thoughts will be greatly appreciated.
Thanks!
Clay
