Hey everyone,

I’m wrestling with a stubborn issue on my 1998 Ford Ranger (2.5L SOHC, manual transmission) and I really need some expert eyes on this. I’ve been systematically diagnosing a string of driveability and cooling symptoms that are tying me in knots, and I want to lay out the whole story to see if anyone has run into this specific combination before.

The Core Problem: Driveability & High-RPM Drown Out

The truck drives okay under light load, but it has a noticeable sputter/stumble under hard acceleration. The real nightmare happens at higher RPMs. If I rev the engine up to around 3,000 RPMs and then chop the throttle (completely release the gas pedal on deceleration), the engine completely chokes out, drowns itself, and stalls.

When tracking this live on a scanner, the second I let off the gas at high RPMs, my short-term and long-term fuel trims bury themselves deep into the negatives. It’s severely over-fueling right before it dies, and the tailpipe smells heavily of raw, unburnt gasoline.

The Cold Start Symptoms

On a cold morning start, I get a distinct, loud valvetrain tap/clatter from under the valve cover. Along with the mechanical tapping, the exhaust throws out a heavy cloud of white smoke. Crucially, it does not smell sweet like burning antifreeze; it smells strongly of raw, vaporized fuel. The smoke lasts for about a minute at idle, clears up as soon as I get the truck moving, and the tap fades away once oil pressure builds. The spark plugs look healthy (tan/brown carbon)—none of them are wet with fuel or scrubbed white by steam.

The Disappearing Coolant Mystery

I am slowly losing coolant, but it’s a total ghost leak.

The passenger floorboard is completely dry (heater core looks good).

The exterior of the block, the radiator seams, the water pump face, and the heater control bypass valve are all bone dry.

The leak is incredibly slow—I topped the reservoir off two weeks ago, and it took those full two weeks to drop near the bottom of the tank.

Interestingly, this leak only woke up after I recently replaced the water pump and the thermostat. Before the swap, the truck was running really cold, and the coolant level stayed completely full. The second I got a fresh thermostat in there and brought the truck up to full operating temperature and normal 16 PSI pressure, the coolant started slowly vanishing. (I just threw some K-Seal "queso" into the radiator to see if it locks down an internal exhaust port crack or a minor intake gasket weep).

Current Diagnostic Theory

Since the head gasket seems intact (clean plugs, no milk on the dipstick) and the injectors are sealing tight, my primary suspect right now is a skipped or stretched timing belt. My theory is that the valve events are slightly out of sync. When I chop the throttle at 3,000 RPM, the high intake vacuum combined with sloppy timing is causing the airflow to stall out while the computer dumps fuel, drowning the engine and tanking the fuel trims. It would also explain the cold-start misfire/smoke and the lifter noise if the cam is sitting out of index.

I’m about to unbolt the valve cover to inspect the timing gear marks at TDC and check the rocker arms for loose hydraulic lash adjusters.

Before I tear into it, has anyone experienced a single tooth skip on the 2.5L causing this exact high-RPM decel flooding issue? Or is there a sneaky sensor anomaly (like a jacked-up MAP sensor or a shorted wire) that I’m completely missing?

Any insight, advice, or sanity checks would be massively appreciated!

This layout gives them everything they need to know—from the exact engine model to the fuel trim data and the exact timeline of the coolant leak. Copy and paste that right into the forum thread and see what the old-timers say!