Why is my Jeep Comanche losing power or idling rough when cold?

Short answer: If your Comanche is weak going uphill, gets poor mileage and smells of gas after you stop, pull the vacuum hose off the fuel pressure regulator: fuel coming out means the regulator diaphragm is leaking and dumping gas into the engine. Members also said to check that old recall work was done. For a rough cold idle, a coolant temperature sensor (CTS) out of spec can cause it; one owner's mechanic blamed an injector instead, and both may be problems.

Symptom: It downshifts but won't pull up hills, struggles with a trailer, gets about 12 mpg in town, sputters with a slight vibration under load, and smells of gas after you stop. Or it idles rough when cold, then smooths out once warm.

What to check first

1. Pull the vacuum hose off the fuel pressure regulator with the engine off. Gas in the hose means the regulator leaks.
2. Check the coolant temperature sensor resistance against spec.
3. Run a VIN recall check for open recalls.
4. Check spark plugs and wires for misfires under load.
5. Have injectors tested if one cylinder misfires.

Likely causes

1. Leaking fuel pressure regulator. Explains a gas smell, poor mileage and low power.
2. Coolant temperature sensor out of spec. Can cause a rough cold idle.
3. Faulty injector. The mechanic's diagnosis on the 1988.
4. Ignition misfire under load. Fits sputtering and vibration.

What it usually costs

No owner posted prices.

Is it safe to keep driving?

A fuel smell is a fire risk; have it checked before more driving. Low power makes towing and merging dangerous. Verify with a mechanic.

DIY or shop?

DIY-friendly: the regulator test, sensor checks and replacements are simple on these trucks.

Recalls, bulletins and complaints in plain words

• Related recalls: NHTSA lists these recalls touching this area of the Jeep Comanche: 90V177000 (1989-1990 model years, vehicle speed control). Check your VIN at nhtsa.gov/recalls. If your car is part of this recall, the dealer fixes it for free.
• In plain words: on some 1989–1990 Comanches, a fault in the throttle position sensor (which tells the computer how far the pedal is pressed) can cause high idle right after starting. If your truck is part of this recall, the dealer fixes it for free.
• NHTSA complaints: 10 keyword-matched complaints for Jeep Comanche 1986–1992 (of 51 unique complaints queried, NHTSA complaints API by model year, queried 2026-10-26; keywords: power, stall, idle, rough, hesitation, fuel, smell, sputter). This counts complaints that mention these words. It is not a defect finding.

What owners report

• In 1987 Jeep Comanche Pickup - Loss of Power on Car Talk, an owner said the Comanche downshifted but wouldn't pull uphill, was useless with a trailer, got about 12 mpg in town, sputtered with vibration under load and smelled of gas after stopping. One member said to pull the vacuum hose off the fuel pressure regulator: fuel there means a leaking regulator, which would explain the poor mileage and lack of power. Another said to make sure past recall work had been done.
• In 1988 Jeep Comanche - Rough idle on Car Talk, an owner said the Comanche idled rough when cold and was fine once warmed up. The coolant temperature sensor measured out of spec, but the mechanic blamed a bad injector, and the owner wondered whether it had two problems. A member asked for more details about the truck and what had been done.

Quick tip

That vacuum-hose test takes thirty seconds and no tools. If gas comes out, you've found your power and mileage problem.

Vehicle focus: 1986–1992 Jeep Comanche (2.5L four and 4.0L six).

Safety / mechanic note

If you smell raw gas, don't drive it until the leak is found. Verify with a mechanic.