Severe hesitation/stumble problem on full throttle!

[Cynic94rs]

Hi guys/ladies:

I'm trying to diagnose a problem I'm experiencing that *seems* to have worsened over time.

My MX-3, at very slight throttle, will accelerate slowly but smoothly all the way to redline without problems...

But at anything over 1/4 throttle the engine will stumble badly at about 2000-2500 RPM. At full throttle, the stumble/hesitation is so severe that the engine will not recover. It just keeps misfiring/stumbing along. The exhaust pulses from the tailpipe on a cold day do not come out smoothly. The problem does not disappear at speed. The problem persists standing still in N and Park, and in D and Reverse. Take off the gas and it will go back to idle cheerfully.

** Here's what I've already done/tested:

New distributor.

New spark plugs, wires, cap, rotor.

New fuel filter.

New air filter.

No CEL or codes thrown from the ECM.

Fuel injector cleaner run through the gas tank.

PCV valve given "blow through" test.

EGR system given "functional check" with vaccuum pump.

MAF tested & is within specification.

Oil, Transmission fluid ok, not burnt, not milky.

Coolant level low.

Apparent hole in downpipe after precat and before catalytic converter.

Low compression in cylinder 2 (105 psi)

---

I've been checking every vaccuum line I can find and while I've found and fixed one broken connection, (intake to EGR solenoid) it didn't make any difference.

Has anyone else experienced anything like this, and where would you start looking next?

This poor beautiful car needs to be on the road.

[yiorta]

You should also try your fuel pump.

[Cynic94rs]

Well, since I first began the quest to fix this problem, I've done quite a bit more diagnostics & repairs. Here is an updated list of what I've done to date:

New distributor, wires, plugs, cap & rotor.
New coolant temperature sensor.
New (used) MAF Sensor. (made no difference)
New IAC valve.
Cleaned throttlebody of all carbon deposits.
Inspected vacuum hoses.
Timing set to specification.
Fuel pressure tested within spec at idle (around 35 psi at idle; steady around 45 psi during missing/hestitation).
Spark plugs properly gapped.
No CEL codes present.
Replaced cracked hose from valve cover to intake.

I note the following abnormalities:

1) Cannot bring idle below 1000 -- adjustment screw completely in.

2) Engine seems to get hot rather quickly. (cooling fans do operate)

3) Timing light attached to any plug wire shows inconsistent spark on acceleration (irregularly flickering light consistent with missing/stumbling).

4) Odd smelling exhaust.

5) Weak battery. (has drained several times as car has been sitting for 4 months).

-----

I'm starting to suspect that parts of my ignition system are to blame, even though they were all recently replaced. This problem did set in a few weeks after I replaced the distributor and did a full tune-up. (to correct a Crank Angle Sensor code from the CPU). Has anyone ever had a distributor go bad on an MX-3 without throwing a code? I can still get a replacement for the distributor under warranty.

I replaced the IAC valve at the Mazda dealer's suggestion, to no effect. They wanted an arm and a leg for it so I bought a remanufactured one, replaced it myself and saved $600.

Further advice, my MX-3 driving friends? Maybe having a mechanic look it over with a scope could be beneficial?

I'm not out of patience or money yet but I'm getting low on ideas.

[ertaisi]

This sounds almost exactly like the situation I'm in at the moment, except my idle has no problem dipping under 1k. One other specific I've noticed with my engine is that the problem doesn't occur when the engine's cold. It runs like a champ, but then rev it up and bring the oil temp up and it runs like crap.

[freyguy_412]

I haven't had it happen to me, but has anyone checked the coil? The spark issue you describe may be a faulty coil, and I didn't see if that was something you had changed.

Is there any recent electrical work that might be shorting out, causing the battery drain?

[Methos]

Hey man, check that coil that just like freyguy_412 said, and I would exchange that distributor just for "craps" and giggles.... you never know could be a faulty distributor, wouldn't be the first time I have seen it

As for the mechanic, what more could he do that you haven't already done? I would let him throw the car on the diagnosis machine if any thing... that might tell you what it is, and it would save you a lot of labor time for him to "check over" all the work you have already done....
Where I live an hour's worth of diagnosis is only about $60. bucks
Let them throw it on the machine and put the throttle down then the machine can tell them what is going on.....

Well, that is what I would do...Just a little advice

[mamoo]

I'm no expert, but I kind of think the fuel injector should get more attention then a jar of cleaner poured into the gas tank.

[Typhoonk]

have you been losing coolant?

[Cynic94rs]

Thanks, everyone. I appreciate the attention you're paying to this issue, and when I figure it out I will be sure to post a reply so that everyone can share from my frustration.

Answers to a few more of your questions & thoughts:

1) Fuel injectors have never tested bad or anything -- some leftover seafoam was dumped into the tank following using it through the intake to clean carbon deposits. Original work was done on ignition system, intake to try and remove carbon deposits in order to bring the car into spec to pass smog. NOX was high at the time, timing had been advanced, Crank Angle Sensor code was being thrown. When I did a tune up the car passed -- with average results.

2) I cannot tell if I'm losing coolant. Coolant level is low but the car is basically undriveable, so it's never on the road long enough to keep an eye on it. There aren't any puddles of it under the car, so if it is disappearing (and I don't know if it is) the coolant must be going into the cylinders... but I have to say, I hope not!

I think I will replace the distributor just to rule it out.... Isn't the coil integral to the distributor? When I change it should I have to replace the cap & rotor again too?

[Tunes67]

No.. as long as you get a identical distributor.. you shouldnt have any issues reusing your cap & rotor.. I would inspect them closely for damage though prior to reusing them. Grab your multimeter and check the value of your O2 sensor as well (online shop manual will have the procedure for that) one possible scenario.. if your O2 sensor is malfunctioning.. it could be sending incorrect data to your ECM.. causing the ECM to lean or richen up the mixture. Now if its leaning it out.. and you have a injector or two that is not up to snuff.. you could run into an issue similar I would imagine.. something to check in any case. You can also check the resistance on each of the injectors as well (see the online shop manual) this will at least tell you if the injectors coils are in good shape. It wont tell you if the spray pattern is good or if the injector is flowing the proper amount of fuel though. You will need to have them flow checked at a shop to find that out. Have you done a compression test yet? Easy to do and it will reveal if you have any valve sealing issues. Post back and keep us upto date

Tunes67

[Tunes67]

Sorry I didnt notice the low compression in #2 cylinder.. what are the other scores from your compression test? 102 psi is a bit low.. but.. I dont know if thats low enough to indicate valve problems or if that would more likely be compression rings. Do you burn any oil? (white residue on the spark plugs? Not tan residue though thats normal) Post back your other compression scores and I will check with my machinist what would be the most likely issue.

The exhaust pulses from the tailpipe on a cold day do not come out smoothly.

I missed this statement as well when I read your initial post.. I recently had a head job done after discovering a burned valve in my #3 cylinder. I had a very rough idle and the exaust pulses were also not smooth. However.. I had only 40 PSI in my #3 cylinder.. maybe your valve in #2 is burned but not as severly as mine was.. I will get a pic of a couple of burned valves so you can see what they look like. You can have a shop perform what is called a "Boroscope" (dont know if I spelled that right though) which they can remove the spark plug and run a lit camera down into the cylinder to inspect for damage. You can also look down through the spark plug hole at the top of the piston.. look at all 4 cylinders.. you should see a even blackness almost like a powder on top of the piston.. if you see the steel color of the piston through this powder.. you could have a condition called "Washing" going on in your cylinder.. this is where oil and/or raw fuel are unburnt and wash the typical carbon build up off the top of the piston. This is common when you have low compression in a cylinder.

Tunes67

[jimishadow]

you probobly need a head gasket and maybe mill the head,how many miles are on engine?

[Cynic94rs]

Many thanks to all of you for sharing your advice and experiences above. I'm sure those of you who've been following this thread are interested in knowing what was causing that bizarre persistent hesitation phenomenon.

The answer: a distributor with a faulty secondary coil.

Now, why didn't I discover this sooner? Because as you'll recall the distributor was replaced less than four months before the problem began, and didn't fail right away. But that wasn't where the saga ended.

I returned the distributor (#2) for replacement under warranty due to its faulty coil. Then I installed the new (remanufactured) distributor (#3) and began the business of setting it up, only to find I couldn't set timing to spec with it; it was impossible to advance it far enough by rotating the body. Feeling like a idiot, I took the car to a shop. I thought for sure that my work was the problem. What were the odds of getting two bad distributers in a row?

Apparently, pretty good. The shop installed yet another distributor (#4, new and not remanufactured) and the RS fired up right away and could be set into spec.

So the moral of the story is: don't assume that when a part is remanufactured that the company that rebuilt it knew what problem they were supposed to be fixing. Neither distributor #2 or #3 threw any codes to the computer, but they were both bad. In addition, while I'd like to commend Autozone for cheerfully bearing with me and refunding my money in the end, I'd heartily recommend against buying remanufactured parts that come from A1-Cardone, the real culprit in this long and expensive runaround.

Again, my thanks to all of you for your help. Thank goodness the problem wasn't worse.

See you on the road.

[forcedairram]

Do you think you could be leaning out ? I had the same problem and when i bumped my fuel up a hair it just stopped , exept around like 4k rpms with light throttle, and I believe thats the vics opening up.

[black_mx3]

mine is doing that too!!! and i wasnt sure if it was bad compression b/c the guy i bought my car from said that it has compression problems in 2 cylinders...but the car doesn't burn a lot of oil...(just a puff now and again) and runs very quiet! so maby it is the distributer..b/c i was told to check all of those things and iam just completely confused