Toyota Repair: Acceleration stumble, harder starting, & abnormal electrical readings, spark plug wire resistance, insulation resistance

Question
QUESTION: Hello Ted,

I have 2 questions. The first regards what to do about out-of-range ignition system readings found while working up acceleration stumble.  The second regards how to approach diagnosis/correction of the engine performance problem from here.

I have an '87 Toyota P/U with 22R engine, still going strong, and I intend to keep it that way as long as possible.  For quite some time, I've tolerated acceleration stumble, worse when engine is cold.  Also, though it's not hard to start, it's definitely worse than in the past.  In the past, it would not die after starting; now, it dies after every cold start.  It usually stays running after the 2nd start if I keep it revved enough.  I first changed the spark plugs, which made no difference.  Then, I checked the spark plug wire resistance, and found all are below cutoff for replacement.  I decided to rule out any electrical cause before checking the carburetor/fuel system; however, I did replace the fuel filter, which didn't make a difference.  The ignition timing is precisely on, and the centrifugal and vacuum advancing mechanisms seem to work OK.  While checking ignition specs, I discovered several that were not within the range in my Haynes manual.  I doubt that these are related to my engine problem, so I wonder whether I should address or ignore them.  Here they are:

Distributor air gap:          .005" (Spec: .008-.016")
Signal generator (pickup coil) resistance:          183 Ohms  (Spec: 140-180)
Ignition coil primary resistance (several readings):  .02 to .05 Ohms (Spec: .4 to .5)
Ignition coil insulation resistance (2 readings):        5M & 1M Ohms (Spec: Infinity--at least >10M Ohms)

The coil secondary resistance is within normal range.  The air gap and coil's primary resistance & insulation resistance are considerably out of range.  What should I do?

On to my 2nd question.  I feel that this problem is likely a carburetor problem, or possibly related to all that emissions stuff connected to it.  I've never done anything to the carb in the 20 years I've had this truck.  As a side note, about 8 years ago, I had some weird issues with the fuel system that started one day and carried on for several months to a year before resolving on its own.  If I punched the accelerator, the engine would surge to high revs off and on afterwards on its own for a while, as if I were pushing the accelerator on & off quite a bit.  The first time it happened, the engine eventually died and wouldn't start for a while.  I was very busy and didn't get around to having it serviced before it eventually resolved.  I just thought I'd mention that in case it helps at all.  I did replace the fuel pump years ago because it was leaking.  What do you think I should do next?

Thanks much.  Your altruism is really appreciated.  I don't know of anywhere else that I could discuss this with an expert Toyota mechanic!

Sincerely,
Scotty


ANSWER: I would try replacing the coil to see if that helps at all, if it does then you know the problem is electrical and I suspect that the distributor may be the problem, all of those readings are very susceptible to temperature changes and even though a resistance test may be within limits at cold temperatures the readings can be quite different when the engine is warmed up, distributor problems used to be common on these engines, so I think your problem may be there.

---------- FOLLOW-UP ----------

QUESTION: I was hoping to diagnose this without buying expensive parts just to try them out. Regarding the coil/igniter, maybe I could just check resistance readings of a new one at the counter.  If they are also out of normal range, then perhaps I could consider them unimportant in my own coil.  How would I diagnose the distributor as the problem?  Do good mechanics just try replacing parts without knowing whether they are the problem?  Do you recommend that I replace the distributor without any diagnostic test?  Regarding the temperature when taking my resistance readings, since the stumble is most prominent when the engine is cold, cold readings would seem to be the pertinent ones.  Thanks.

Answer
Yes, sometimes it is necessary to substitute known good parts, especially if in past experience electronic components tested good and within specs but they were still the cause of the problem, resistance changes with temperatures, a test that is made on a solid state circuit may look ok when cold but resistance will change when temperature changes, the resistance of all resistances will increase when temperature increases this is basic electricity theory.