'93 mx-6 stumbles when driving

KENLOVESJENN

MEMBER
1993 MAZDA MX5

'93 mx-6, 2.0 li, 237,000 mi.
2 years ago, had "no G-signal" code - replaced ICM. Problem reoccured within 1 year - replaced distributor/ICM.
Ran fine for a while, then abruptly began to stumble when accelerating. Impossible to get speed. Similar feel to ICM problem, however no codes (hard or soft). In park/neutral runs fine, idles and revs without hesitation.
Has new plugs, wire resistance in range, new cap/rotor, new fuel filter, fuel pressure tested in range (increased when regulator vacuum removed), injector coil resistance in range, cylinder compression all over 150 psi (held pressure).
Its just odd to me that it runs so well in park, but in gear - with some gas - it just bogs down. Wouldn't clogged injector fault manifest full time? Or PCM and ignition faults throw a code? (Obd-1)
I work on scissor lifts and booms (aerial manlifts). The electrical and hydraulic systems are fairly complex but ignition and engine controls are VERY simple compared to automotive applications, where I'm not as well versed. My instinict tells me I'm missing something obvious. (?)
Desperate to fix this before the rain starts again (seattle - no garage/carport). Any suggestions for my continued troubleshooting would be greatly appreciated! Thanks!

9 Replies

2CARPRO JACK

MECHANIC
11,533 POSTS

Might be a partially plugged cat converter. Loosen exhaust before the cat and drive it, loud, but will run ok if the cat was the problem

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Thursday, August 9th, 2007 AT 7:50 AM

KENLOVESJENN

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I removed cat. Converter entirely. Converter checks out - could see daylight through it, nothing rattling around when shaken. Engine still stumbles. I'm bummed, that was a very promising lead.
If you have any more suggestions, PLEASE throw 'em at me.

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Sunday, August 12th, 2007 AT 4:18 PM

2CARPRO JACK

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This car have a Mass Airflow Sensor? Could be the problem.

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Monday, August 13th, 2007 AT 8:11 AM

SERVICE WRITER

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Diagnosing without codes it much tougher. Air flow sensor may be a problem as Jack stated, sometimes cleanable, or anything else regarding air flow including blockages (like a plugged air filter), or leaks in the air intake snorkel. Some basics that may be affecting this to check include a Egr valve that may need cleaning, Check to make sure fuel pressure is still in spec when problem is happening, check coil resistance.

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Monday, August 13th, 2007 AT 10:06 PM

KENLOVESJENN

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Both key on/engine off and ko/e on voltage values in range (as per Haynes). Voltage increased smoothly when throttled up. No physical blockage.
I guess I should probably check other engine control sensors (TPS, IAT, CMP etc.), I just assumed there would be a trouble code present with these faults. (?)
After the dist. Went for the second time, I had considered having the PCM replaced on a whim. Apparantly they (NAPA) take the existing PCM & send it to the shop for testing/repairs. Once again, though, I am receiving no trouble codes. Ever heard of a PCM failing w/out self diagnosis?
By the way, I wanted to thank you for replying so expediently. After checking out the total volume of this site, I had hoped for a reply within weeks instead of. Well I shouldn't say how long it took lest everyone expect the same turnaround! With this kinda service, I would gladly donate with every further correspondence rather than schmoozing the mechanics/service writer down at the Mazda dealer.
Thanks man.

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Monday, August 13th, 2007 AT 10:22 PM

RASMATAZ

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My son had that problem w/MX6 2.5 before it blew up. Check the coolant temperature sensor and purge control solenoid valve. Also run an injection cleaner thru it.

One of these fixed the problem, just forgot what it is.

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Wednesday, August 15th, 2007 AT 4:42 PM

RASMATAZ

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Try checking for a leaking fuel pressure regulator.

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Saturday, August 18th, 2007 AT 7:17 PM

KENLOVESJENN

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Sunday, August 19th, 2007 AT 12:10 AM

RASMATAZ

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Thanks for the suggestions. I'll check coolant temp. Sensor. Fuel pressure regulator tested good when I did fuel pressure test. Is there another test other than looking for pressure increase when vacuum line removed? Do you mean fuel, or vacuum leak? What is "purge control solenoid valve"? Can't find anything on it.
In response to "Service Writer", No physical blockages found. Even though MAF checked out on voltage tests, I'm told that all MAF faults may not show up on meter. (?)
I may replace it just to be safe. No other air blockages found. Ign. Coil primary & secondary resistance in range.
EGR valve buried under throttle body, so since I'm taking that off, I'll go ahead and pull the injectors and either take to shop for sonic bath or replace (tested each injector coil).
I guess I could rig the fuel pressure gauge so that it's visible to read the pressure while I'm driving. Is that what you meant?[/Quote:192d2c1125]

Try running an injection cleaner thru the system before you do all that stuff-might work.

The regulator is leaking fuel when vac line removed.

About the purge control valve solenoid-the computer camed up with the code 26 and also for the CTS. I did resistance check the CTS was open change it all the codes disappeared.

You probably need a scope/pressure gauge to nail the pump.

BTW have you tested the TPS/EGR and look for vacuum leak. This engines 2.0/2.5 its a mil of vacuum lines. Lol and its hidden real good like Easter Sunday.

Let me check with the local mazdas boyz maybe they know something I don't

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Sunday, August 19th, 2007 AT 1:04 AM