My 1992 245 210k AT has been having a very random intermittent stumble. It can happen at any speed under any conditions but seems to happen more often when the engine is warmed up and cruising at high speeds 50-70 mph. It feels much like the fuel pump quitting, for 2-10 seconds, then all back to normal. It's not a "missing", it's more a stumbling along until it catches itself and smoothes out. It may happen a lot for a few days then not at all for a few weeks. No relation to how much gas in fuel tank (usually keep it above half full) or weather.

It ran perfectly until I had RH engine mount, transmission mount and intake manifold gasket replaced. At the same time the TB was cleaned and adjusted and flame trap replaced. All this took care of the shaking at idle but that was when the stumbling started, at first along with stalling and hard starting. No OBD codes. This is what was done:

1. Checked fuses (good & clean). ECU reset. Problem recurred after 50 miles.
2. Replaced crank position sensor and fuel pressure regulator (was leaking). Problem recurred after 200 miles, however, no more stalling at idle, just stumbling randomly.
3. Replaced fuel pump relay - no change.
4. Replaced AMM w/good used one, cleaned airbox, replaced air filter.
5. Deoxidized/greased all engine grounds, battery cables, ignition amplifier, cleaned spark plugs, lubed and adjusted accelerator & kickdown cables, TB linkage, pin-tested all engine sensors at ECU (tested good), tested ignition coil (good), fuel pressure (good - of course it could drop randomly?).
6. Two more vacuum leaks found and fixed.

So engine bay is nice and tight and very clean, no loose connections, vacuum leaks, etc. All was well for 600 miles when it started stumbling AGAIN - same problem. So this week we switched the -561 white label ECU with a 951 ECU and it seems fine but I felt a couple of slight stumbles already.

No OBD codes or CEL. Everything tests fine when tested. No tach for me to check when it's doing what it's doing. Fuel consumption is still slightly up even with all the recent tune-ups and repairs, but improved since AMM, now 19 city/25 hwy (used to be 22/27).

Previous (related) repairs were:

1. O2 sensor & idle control valve replaced 4k ago.
2. Plugs, wires, cap, rotor, fuel filter 9k ago.
3. In-tank fuel pump, pre-filter 23k ago.
4. Main pump 9 years/80k ago.

I shall add, all parts are OEM, no aftermarket.

I am about to throw some more parts at it starting with the two oldest the ignition coil and fuel pump but those would be guesses.

Any thoughts are appreciated!
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1992 245 187k