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Hello SD Forum,
I am trying to diagnose a High Idle issue after installing an aftermarket TBI kit on my 1987 Jeep AMC 4.2l. I have the vehicle connected to TunerPro RT but I’m not an engine diagnostics pro and need some help knowing how to read and interpret some of the data. Here’s some vehicle history and specifics about the issue I’m having.
EFI System is a Howell JP-1 CA kit which is based on 1988-1993 GM K1500 Light Duty Truck w/ 4.3l V6. The system is a TBI with GM 1227477 ECM. Sensors are a TPS, IAC, MAP, Knock, VSS, ECT, and a single narrow band O2. This system does not control engine timing. HEI distributor has mechanical & vacuum controlled advance.
ISSUE IS AS FOLLOWS
Intermittent high idle. Sometimes it idles at 2300, 1200, 1800, and other times it idles normally at 750. If I start the car cold it idles fine then warms up fine and drives normally. After that, when I start the car the second time, the idle has a mind of its own. Sometimes the car warms up and the idle becomes normal. Other times it doesn’t matter that it warms up. I simply re-start the car and that fixes the problem.
I’ve checked for vacuum leaks (carb spray method) and replaced all hoses and fittings and put clamps on all connections. I disconnected the IAC valve and the car idles normally, so I replaced the IAC with no change. Howell told me to replace the ECM so I did that and no change. I’ve backprobed the TPS and it’s working normally. I tested the brand new EGR when it was idling normal using a vacuum gun and the idle doesn’t drop like I’d expect. In addition, this motor has always had low manifold vacuum showing 15 inches of mercury at 750rpm.
Anybody know what the next step would be? I’m new and still learning this system and how to use the scanner graphs and O2 reading etc. I have TunerPro RT file exports to share if needed.
Is the IAC a 4 wire stepper motor that GM uses? My guess is yes. Just an FYI when you unplug a stepper motor it stays in the same position.
Anyway, you want to be thinking inputs here.
TPS, ECT are your two big ones. Then power steering pressure (switch) and park/neutral (switch) and A/C (switch)
I have seen as little as a .1v change (increase) on a TPS signal cause a very high idle speed. Why? Because the ECM thinks you put your foot on the gas pedal and it is preparing for the decel event so it opens the IAC. Look very very closely at the TPS
- Yes it is the 4 wire that GM uses. I’ll post the system specs at the bottom of this response.
- TPS checks out fine. I looked very close with a Snap-On Vantage Scope.
- ECT however had multiple issues that I’m still working on solving.
1. CTS tested bad so I pulled it out and bench tested it using a Jet Boil stove and the thermometer and it tested GOOD! I graphed both tests against specs for that particular sensor on the same piece of paper by hand. Reinstalled the CTS. Connected a multimeter and observed voltage from CTS signal wire to (-) battery terminal ground. Signal was very unstable bouncing up and down fairly quickly all over the range between .8 - 4.5v. Any advice on how to diagnose this?
2. I decided to make sure that coolant had a clear path to the CTS which is located in the intake manifold on an 87 Jeep 4.2l YJ. Found that the previous owner left a rubber plug stuck in the outlet port of the water pump which goes to the intake manifold. So I removed it and retested the CTS but No Luck! Signal still bad.
3. Downloaded a coolant flow diagram from ALL DATA. Found that the loop from the water pump to the heater core then to the intake manifold was not run correctly. Ordered new hoses. I’ll install those this week.
Given how rapidly the voltage changes in the CTS signal wire it makes me think this is an electrical issue. If it were a coolant flow issue I would expect to see slower changes in the signal wire as the thermistor expands and contracts. Correct?
Any help to further solve this issue would be greatly appreciated.
Thanks,
Brent
Howell JP-1 EFI System Description
This is a very common 1987 - 1991 GM 4.3L V-6 Throttle Body EFI System with a special Calibration Prom so it is compatible with the Jeep 4.2L 258cc I-6 Cylinder Engine. It uses the following components.
1. Two barrel TB unit with integral TPS and Idle Air Control
2. ECM GM Part Number 1227747
3. Cal-Pac (L-6) for “Limp Home” operation inside the ECM
4. MAP Sensor GM PN16165793
5. Engine Coolant Sensor for ECM control GM PN25036979
6. Single wire O2 Sensor GM PN25166816
7. Electric Fuel Pump, High Pressure, In-Line HED PN FP257
8. High Flow 3” Dia. Fuel Filter Fram PN G3802A
9. Fuel Pump Relay GM PN 14089936
10. Vehicle Speed Sensor HED PN VSS263
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