Help - failed CA emissions, misfire??

[surfstar]

My car - 1993 Mazda MX6 2.0L ~209,500 miles

Last test two years ago was fine. Today it failed for HIGH HC at idle. It was 1000-2000; pegged the meter @ 2000. CO was 0% @ idle too, which was weird. At 2500rpm it was close to the passing limit; could have gone either way.

About 3 weeks ago, I noticed a rougher idle and ocassional dip in the idle by 100-200 rpms. My idle has always been smooth and steady. I ran two tanks of Redline SI-1, no change. Changed the plugs (old ones had 7k miles only), no change. Disty cap/rotor and fuel filter were changed at 203k. Wires were done at 179k. Cleaned the IAC valve, no difference.

I tried pulling the wires from the plug and from the disty to try and locate a misfire. All cylinders made the same difference - drop in idle and rougher. Each plug lead had spark at the end. Air filter is fine. Distributor ("disty") is 1.5 years old.

I'm stumped. Next will be to check compression. I've always run syn @ 5k OCIs, so I think the engine should be fine internally. (I was going to start ARX on my next OCI, after smog)

Not sure if its a full misfire now. I don't know what else to check. Its OBD1, so it doesn't help. Runs great at all other rpms, the "miss" can be felt up to ~1500 - MPG is unchanged. Thanks for any tips or ideas.

 

[Quest]

I would go through all the ignition stuff once again to ensure nothing is bad (going bad) or misbehaving.

-distributor cap and rotor: did you replace it with OEM Mitsubishi dist cap and rotor? Have you also check to see if your distributor unit seal starting to leak?

Another thing worth nothing for is the distributor cap to housing seal. Have you replace it yet? (this 5bucks o-ring typically lasts about 6~8yrs before it needs to be replaced, otherwise, moisture seeps into the distributor causing spark leaks and misfires.

-have you check to see if your spark plug cables are at fault? If you haven't replace them recently/regularly, do so at your earliest convenience and use NGK blue. don't bother with Bosch for I have gone through 5+ sets and they are no better than those crrrapy autolite orange stuff.

-have you pull your spark plugs to see if there's anything wrong? Any shorts or similar?

-have you try unplugging the MAF sensor plug and then re-seat it to see if it helps?

-have you clean your IAC valve? If yours has a rubber cap on the top of the throttle body that permits you to adjust idle, have you try unscrewing that screw and then clean it with CRC throttle body cleaner?

-have you check to see if you have any CEL codes? Have you try resetting the computer to see if it gets a bit better?

This is usually a good start. Plse post back to discuss further (assuming that all other adjustments such as idle rpm, ignition timing, etc. remained the same before this happened).

Q.

p.s. you said you have your distributor replaced not too long ago. did you replace it with a quality rebuild or a brand-new unit? (Cardones are mostly rebuilds and quality is so-so). If you can find a wrecker original distributor unit (should be Mitsu if memory serves), I would recommend you to stick it in and give it a try. I have very poor luck when it comes to Cardone rebuild distributors on my client's car, citing ignition module goes bad/misses after several months to approx. 2 yrs, and distributor shaft seal leaks after 1 yr of service), etc.

 

[surfstar]

UPDATE: I tried all the normal things/suggestions above, and all tune-up components are fairly new; disty is a new non-remain unit. IAC, MAF cleaned, etc.

Took it to a shop today and they diagnosed a leaky intake manifold gasket :doh: I wish I would have tried to listen for that. $70 diagnosis

Their quote for R&R the gasket - $455.57

umm, thanks, I think I'll go buy the $5 gasket and spend the 3 hours doing it myself.

 

[Cutehumor]

a used oil analysis could have found it.

 

[Trav]

As would a cheap vacuum gauge,the vacuum gauge is one of the cheapest most overlooked tools for diagnosing modern emission control systems.Mine is still in service 30+ years and has no dust on it.

 

[surfstar]

A vacuum gauge is needed in my limited tool selection for sure. Knowing how to use it would help even more.

Quote:

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a used oil analysis could have found it.

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Check the used oil analysis section...I had one done on this actually. ~3mo used oil analysis and the last 1.5 months have had the stumble at idle, so I'm assuming that's when the gasket went bad.

Here's why I hate taking my car to a shop for diagnosis: after I pick up my car and get some lunch, I notice the shifter feels weird...its picking up lots of engine vibrations and feels like its hitting something when shifting some gears. I figure the heat-shield must be in the way. Stop by the shop and ask if they looked at anything under the car by change; nope they didn't even lift it.

Drove to my Mom's for her Bday, then threw the car up on stands and crawled under it around 8pm. Linkage was fine under the shifter...lets see how it looks going into the transmission...hmmm what's this thing here I see? ...a freaking mag-lite! I solved my clunking/rough shifting and got a new flashlight ta boot!

 

[c502cid]

surfstar....freebies are good! I took one of my beater cars in for a "free oil change" coupon I received. When I looked under it after I got home there was still a snap on wrench on the oil drain plug...and yes the drain plug was tight.

 

[surfstar]

Well I replaced the $5 gasket on Monday, and it passed smog today!

In case anyone cares, here's my smog data - looks like mileage does increase emissions over time...

Bought the car and smogged it - unknown service history; well maintained since then with 5k mi OCI and syn, plus on-time regular tune-ups.

6-1-01 83k mi / 2-5-05 162k mi / 2-16-07 212k mi / MAX

IDLE:
HC ppm- 8 / 10 / 23 / 100
CO %- 0.01 / 0.00 / 0.00 / 1.00
CO2 %- 15.1 / 12.8 / 10.1
O2 %- 0.2 / 3.3 / 6.3

2500 RPM:

HC ppm- 8 / 12 / 21 / 130
CO %- 0.00 / 0.00 / 0.00 / 1.00
CO2- 15.1 / 14.5 / 12.7
O2- 0.2 / 1.1 / 2.7

hmmm, can't find my 2003 report. I also have a very slight exhaust leak after my cat, so that probably accounts for the increased 02 this time. Also I'm 200 miles into my first Rinse phase of ARX, I wonder if that helped or if it was too soon. Oh well, I'm good for another two years