jumping/lurching/bogging below 2500rpm, smooth over 2500

[owb]

Hello. Great forum; have used it many times to solve other issues.
I looked through this site and have seen some similar threads but nothing that really matches my current issue.

1993 Sidekick, 16v EFI, Manual, Mileage: 201,000

When accelerating, especially when accelerating hard, the engine bogs/hesitates/lurches/jumps and the result is very low power and RPMs unless you feather the throttle lightly to slowly build RPMs without putting lots of load on it. This only happens until about 2500RPM (in any gear). Once above 2500, you can accelerate as hard as you want and the problem doesn't occur.

It seems to only happen noticeably when under load, and it is only faintly detectable when parked and revving it in neutral.

No check engine light. Shop said timing was fine and they didn't report any codes.

I do have a cracked exhaust manifold,upstream of O2 sensor but just had welded a few months ago and it seems to holding, as O2 sensor not complaining.

Also, it does burn a bit of oil and has a decent cloud on startup which I believe to be worn valves, but this has slowly been building for years and has not changed with this new problem.

Finally, I did have the dreaded issue with the crankshaft keyway getting wallowed out due to sheered/loose crankshaft sprocket bolt, and I rebuilt the keyway and rebolted it 5 years ago and it has been running very well since, until current problem. Perhaps that is coming loose again?

Interested in ideas before I blow a weekend or two chasing an unknown problem.

Thanks for any input!

[fixkick]

That is a set of odd symptoms. (demon like) (i guess hot engine only)
bucking is mostly spark.
but why and which part?
spark wires ok?
with the DIag jumper planted, and 12s going, drive it. get it to buck see if 12s just beats out, no drop in cadence of 12s, and no new codes pop up then clear.
by cadence mean, 12,12,12,12,12s no change'
not 12,12,12,41,42,12, 12......
or 12s die for a time? goes dead, no flashing codes at BUCKING? (implying power loss, even loose fuses FI, or IG coil)

bad grounds? in the engine bay. engine block grounds? ,distributor base grounds wires at bolt mounts?
spark coil going bad slowly.
id do spark plugs, wires ,cap rotor and cross fingers. if no good now, COIL. spark igniton coil on fire wall.

fuel loss, is usually smooth not bucking violent.

hope you find it easy, cheers and have great Sunday.

[owb]

Thanks for the response. It happens when hot or cold...no obvious difference in the symptoms when cold vs hot.

Wires were changed about 6 years back, look fine now, and sparks looked good too but they are getting old so I'll replace those and see. I looked in the distributor cap (it's new as of the last 6-10 years) today and it seemed really clean in there but didn't dig deeper.

I should specify it is only rarely a' violent' bucking, and is often just an erratic choking that can come in little quick sub-second spirts up to longer hesitations of a few seconds.

I'll get my hands on a diagnostic and check 12s. Thanks.

[fixkick]

sounds like spark bad (seems)
the symptoms are not clear to me. (under all driving condidtions)
hot and cold, ok both and super good clue there.
if not spark what is it. (plugs,cap,rotor.wires set ) I run mag wires, to make spark way better on this WEAK system.
the spark gap must be 0.028" not , .045" as they come out of the box (as all do) is this correct,?

the events happen all the time or is it random.
idling parked
driving only , my guess yes.
driving flat grown steady right foot, ? i guess no.
accelerating , yes. even on flat ground and on hills.?
wot, do you have full wide open throttle power on hills, in all gears,that normally can do this.???


so far seems like low fuel pressure, (or bad maf of the maf never cleaned)
bad FPR (may are now bad this old , vast posts here on this 1 part)
weak pump
weak MAF.

if and only if spark is ok.