intermittent miss

[91MMY59]

here is what i have:
91 mighty max w 2.4l and 5 speed.just bought it from owner of 13 years.
truck idles ok,has what i would say is the typical mistubishi studder at idle.I have owned eclipses and starion/conquest and never had one to idle perfectly except when I upgraded the MAS from a newer car.
anyway,driving down the road,it will stumble,kinda like a quick booble like it wants to misfire.this may go on for a mile or 10 then it will just pick up a little more power and run fine.May run fine for a mile or 20 then go back to the stumble.
On my old starquest cars,i would be looking at the vac advance first.
on this one,cause we had another do the same thing,we changed the fule filter.This did not clear it up at all.
truck has 190k.was told new water pump/timing chain etc 60k miles ago.new clutch 30k miles ago.Wires look new and the plugs look great,NGK.
Just wondering if anyone else has had this happen.I am leaning to maybe cat trying to clog up,mas air or possibly fuel pump losing pressure or FPR

[camoit]

Sounds like there is a loose plug or bad ground in the electrical system.

[91MMY59]

all seems fine with plugs and grounds

[camoit]

I'm leaning toward a pick up coil in the distributer or something along that line. Bad dropping resistor, something electrical. There must be a trigger like a bump or heat, something.

[91MMY59]

Im gonna check inside the distrubtor tmrw.The cap is new alone with wires and plugs,air filter.looks like someone was trying to find it also

i also unplugged the MAS,it didn't make any difference,ran fine with it unplugged

[camoit]

Well maybe one of the EFI guru guys will pop in here on this one. Like Droppedmitsu or LSRMike

[DroppedMitsu]

Just listing troubleshooting tips from manual:
Vacuum leaks, improperly gapped/bad plugs, cracked distributor cap, moisture under dist cap, faulty coil, clogged fuel injectors(seen a few people in another site with dirty injectors giving them trouble), bad fuel pump. You probably want to pull the egr off and clean all the carbon out with carb cleaner and also pull the intake off and clean out the egr passages as they're probably clogged with crap.

[LSR Mike]

If you unplugged the MAS and it didn't change anything I would suspect your running in LIMP Mode, or Open Loop Mode, which is the mode which lets you drive it till it gets to the shop. The MAS does Airflow, air temp and Barometric pressure all which are used in closed loop mode. failure of any of these can trip you up.

page 6-26 of the Haynes manual (available in the download section) has the output codes from the OBDI plug to tell if the sensors are bad.

But as Dropped Mitsu says, clean things up first, you may be thinking it just ran the same, with the MAF unplugged.

[lush90]

It does sound a lot like dirty injectors. My '91 idled and accelerated up to about 30 mph just fine but above that ran really roughly. Pulled the injectors (which is easy to do) and had them tested: 4 were like 90% closed and the other 2 were like 50%. Had them ultrasonically cleaned, reinstalled with new o-rings and it runs like a top. Don't waste your time on the injector cleaner additives, I tried about a case worth in the few months before I finally had them cleaned. Well worth the $30 per injector cost.