1991 Explorer Surges At Highway Speeds

[wildbill23c]

Getting ready to help diagnose an issue with a 1991 Ford Explorer that surges at highway speeds. It has a 5 speed manual transmission, manual hubs, manual t-case, has the 4.0L V6 in it. Only surges at highway speeds, you can drive it around town and its fine.

My thoughts are a vacuum leak, dirty MAF...not sure what else to check, any tips would be appreciated.

 

[RonD]

Change fuel filter.

After engine is warmed up unplug the IAC Valve, RPMs should drop to 500 or engine may even stall, either is good it means no vacuum leak.
If RPMs stay above 700 then there is a leak, start checking hoses with IAC Valve unplugged.

TPS(throttle position sensor) could be wearing out, need a volt meter and sewing pin.
TPS will have 3 wires
Upper is 5volts from computer
Lower is a Ground
Center wire is the one you want to test, signal return to computer

Turn key on
Set volt meter for DC Volts
Push sewing pin into Center wire
Ground volt meter
Test voltage on Center wire
Throttle closed should be .69 to .99volt, under 1 volt
Open throttle slowly and watch the voltage, should see a steady increase, no jumping or dropping, if voltage jumps around replace TPS.
At Wide open voltage should be above 4.5volts


Although unlikely there could be a problem in the exhaust, a blockage, that effects flow at higher sustained RPMs, Vacuum gauge can often detect this.
But this is usually described as a loss of power at higher speeds, not a surging.

 

[wildbill23c]

Thank you Ron, pretty much the items I was thinking were possibly wrong. Going to let the guy know to check these. I know they replaced the in tank fuel pump a while back although that doesn't mean the pump isn't bad, but not as likely as if it weren't new.

Could dirty or bad fuel injector(s) possibly cause this as well? Lower speeds now problem, but higher speeds causing the surging?