Are you getting a stutter around 2000-3000 rpm when accelerating hard, do you get cut outs when idling
after a long run. If yes, try changing the spark plugs, they're cheap and easy to replace.

The plugs in my car were the NGK BKR6E`s which are special order everywhere, a quick check on the
computer under 1991 Isuzu Impulse RS gave me Ac Delco RapidFires 5`s for $3.99. They have
platinum tips for 100,000 mile use ! They are gapped at 1.143mm

Heres Some notes from Doug.

--do not change plugs with a hot engine--you will burn your mits
--you can use the allen wrench in the "trunk toolkit" for the cover bolts
--do NOT let anything drop into the cylinder unless you want to take the
engine apart!
--Do not trust factory set gaps--be sure to verify the gap of the new plugs
with a good gap tool before you install them.
--add a single drop of engine oil to the threads of the new plugs to help
them in--this will keep them from getting stuck in place during future
removal too.
--be sure not to "cross-thread" the plugs as you put them in.  Again a costly
($$$$) repair. Do not use a wrench to thread them for this reason!!
--Use either a torque wrench (boy I need to buy one of these) to tighten the
plugs or do it by hand ever so gently (just enough to get the seat ring
flat---similar to tightening your oil plug bolt, but less force needed)

Here goes.

Tools I used were allen key, sparkplug remover, sparkplug gap checker.

Pop the hood.

Locate the black cover on top of the camcover. Its easy to spot right in the middle of the engine,
should have 4 red wires coming out of the right side, if they haven't been changed for aftermarket.

Remove the 6 allen bolts holding the cover in place.

Remove the cover.

On the original plug wires, they're numbered left to right 1 - 4 and the length is adjusted so its
obvious which way they go, but just in case mark sure you`re sure where each goes.

Remove each of the plug wires, hold the top of the connected use two fingers under the wire, and
sticky out bit, and pull firmly. It ought to pop off. Repeat for the remaining connectors.

Remove each of the spark plugs in turn, check them as you pull them out, they ought to have a gap
of 1.4mm, if they are 6E`s.  If the gap is much bigger then you've likely found the problem, also check
the condition of the plugs. Make sure you remove the gasket (the ring around the plug) too, failure
to do so might cause you to install two gaskets, which in turn can lead to knock.

Normal Spark plugs will have a grey/brown dusty residue. Sometimes get other colours such as
red or yellow, usually caused by engine additives .

Normal

Carbon fouled, (soot) overrich carb mixture, leaky fuel injectors, amongst others causes this

Carbon fouled

Powdery deposits, usually ok unless they glaze, which reduces the effectiveness of the spark plug
since the glaze becomes a conductor and shorts out the plug.

Powdery deposits

Detonation, Ignition timing advanced too much, fuel octane too low, should be 92, This is *very*
bad for the engine. Often called ping or engine knock. Don't use cheap gas, it isn't worth it. I can't
stress enough how bad detonation is.

Detonation, Yowsers.

Pre Ignition, cooling system isn't, sticky valves, over lean air/fuel mixture. Also can be caused
by incorrect spark plugs (heat range too high) and installation, over heavy high speed usage,
i.e. track use.

Pre Ignition

Replace each of the plugs with the new ones, give them a nice just over finger tight fit and replace the
connectors fit them firmly in place, replace the wires in the clips try and keep the same layout for the
wiring don't twist them or unduly cross them, start the engine,  running ok ? good. If not turn it off,
check the cabling for a snug fit, I noticed that the plugs I used had a bullet shaped connector which
meant that the fit wasn't quite as tight as the NGK's, but they're still good. Rough running might be a
gap problem, you ought to have bought pre-gapped plugs, incorrect handling of them could have
caused them to be bent during installation, take them out and check the gap against the recommended
gap.

Replace the black cover, and 6 allen bolts.

You could check them after a week or so, make sure that they are still tight fitting, and that theres
no odd deposits etc. Though it might take more than a week for some of them to show.

Have fun

Charlie

Thanks to Bill Luton of Bill`s IsuzuPerformance for the initial suggestion for the cause of the problem.