1992 E350 with 351 W engine. All at once, on a trip, it started running bad. Was fine up to then. (only has 44K miles on it) No power., got us home, barely. Did all kinds of diagnostics on it, tried 3 other computers, all new ignition components, new cat. converter, Had it to 2 different shops, they cannot find the problem. I was told the compression tests fine. The second shop said on a leak-down test, cylinder 4 has a leaking intake valve, and cyl. 8 has a leaking exhaust valve, (or vice versa) How was the compression tested OK then? The second shop had it for months! (irrelevant) I had to pay those shops enough to buy a replacement engine by now, but it looks like a very tough job to R&R it! I am near the Charlotte Motor Speedway, I would think there would be a good Ford mechanic around here, but they don't want to work on anything this old! They want huge money to switch it out for me!

 

Two things to try the easiest is replace the MAF sensor and the second is to remove the valve covers and look for a bad valve spring. A bad valve as spring will cause all sorts of miss fires on port fuel injection motors.

Denny

 

A slightly leaky valve or two with good compression won't cause "All at once, on a trip, it started running bad" and "No power., got us home, barely".

Also, a leak-down is over time, while compression is in the moment while cranking the engine. You can have good compression while having slightly-leaky valves.

You're going to have to go into more detail here:

- Specifically what new ignition components?
- Why was the cat replaced? Was it diagnosed as plugged?
- What were the compression numbers?
- What does "running bad" mean? Misfiring? Shaking? Knocking? Noises? Etc.

 

It takes three things for ICE - compression, fuel and spark. If it has those three things, it is likely an ignition / electronic control part that crapped out, especially with sudden onset of running but no power.
Here's a troubleshooting list, courtesy of ChatGPT, when I asked this question: 1992 Ford 351-W V-8 suddenly loses power under load. What is wrong?

ChatGPT Answer:

Possible causes (prioritized by likelihood and ease to check):

1. Fuel delivery
- Clogged fuel filter or weak fuel pump → low pressure under load. Check fuel pressure at rail (spec ~4–6 psi for EFI of that era; carb models: ensure bowl fills).
- Carburetor issues (if carbureted): clogged jets, stuck accelerator pump, or collapsed float needle → lean under load.

2. Ignition
- Weak spark under load: worn points/condensor (if distributor points), bad coil, cracked plug wires, fouled or worn spark plugs, or a failing ignition module. Inspect plugs and wires; check coil primary/secondary and dwell (points) or module operation.
- Distributor advance stuck (mechanical or vacuum advance) → wrong timing under load.

3. Sensor/fuel metering (if EFI)
- Bad MAP/MAF (if equipped), TPS, or fuel pressure regulator causing lean mixture under load.

4. Air intake / vacuum leaks
- Large vacuum leak can cause lean condition under load. Inspect hoses and carb / EFI gasket.
- Restricted air filter or intake obstruction is less common but possible.

5. Timing
- Timing jumped (loose timing chain or slipped gear) → loss of power, especially under load. Check timing marks; excessive chain slack or worn sprockets.

6. Exhaust restriction
- Partially clogged exhaust or collapsed catalytic converter (older pre-cat trucks may have muffler/exhaust blockage) → power loss under load and increased backpressure.

7. Mechanical issues
- Low compression (worn rings, burned valves) — does problem occur at all RPMs? Perform a compression or leak-down test.
- Valve train problems (broken spring, valve not seating) causing sudden loss under load.

Quick checks to narrow it down (do in this order)
1. Listen for misfires; inspect spark plugs for color/wear.
2. Check ignition coil, wires, and distributor cap/rotor.
3. Check for vacuum leaks and condition of vacuum advance.
4. Check fuel pressure.
5. Verify timing at idle and with slight revs; watch for jump/misalignment.
6. Scan for any codes if EFI or electronic ignition present.

 

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