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Old 08-22-2015, 07:12 PM   #1  
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MAF Question.

So, last night I took my '85 IROC to cruise night, with a couple of friends. I noticed my car was running rougher than normal, but I haven't driven it in a while, and simply chalked it up to that.

We got on the tollway and were caught by bumper to bumper rush hour traffic. Just to add some info, my fuel pump was replaced by the previous owner and has always whined whenever I go below 1/2-1/4 tank. That day, it was screaming like a banshee. After about 30 minutes in traffic, my car died and I pulled it over on the shoulder. Suspecting heat/pressure in the tank and assuming this was my problem ,I took off the gas cap to vent the tank, let the car cool and it started it up after 10 minutes, went about 1/2 mile and died again, but this time with a "check engine" light. Did the same thing, restarted and the check engine light turned off and did not reappear and thankfully was able to pull off the toll way, and filled up with gas so I could shut up that loud fuel pump.

My friends behind me noted that when I'd get on my car they'd see a brown cloud of smoke and a strong sulfur smell. The car's exhaust note sounded different as well. I'm thinking that my cat is fried.
Got the car home and wanted to see what code was stored, jumped the ALDL and got code 34 - Mass Air Flow. This code has never appeared either before or after this incident, nor have I ever suspected the MAF of malfunction. But I do realize a bad MAF can take the cat down with it sometimes.


I guess my question here is, it's pretty obvious my cat is toast, but what do we all think about this Mass Airflow situation?
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Old 08-23-2015, 01:14 AM   #2  
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Re: MAF Question.

My suspicion is that a clogged cat created unusually high vacuum, triggering a code 34. Does my theory make sense?
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Old 08-23-2015, 10:08 AM   #3  
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Re: MAF Question.

The exhaust smoke and odor is indicative of an overheated cat. It may be melted down at this point, or maybe not. That could be a symptom of a larger problem, or may be the only problem. Ultimately, installing a new three-way cat converter may address that symptom. It will be important to determine why it overheated to prevent damage to any replacement cat.

Your analysis is good and shows sound reasoning, but consider some other factors. Please pardon the forthcoming missive, but it may be helpful.

The operation of the MAF is really not dependent upon vacuum. The MAF sensor actually detects a controlled heat source being dissipated to the intake air passing through it, and thus the rate of heat loss to the air. As such it is fairly well compensated for altitude. density, humidity. The rate of heat loss to air is fairly constant as a function of the air mass instead of volume regardless of all of these factors, so the concept of the sensor is quite clever. The engine management system only needs to know the mass of air to be able to apply the correct mass of fuel.

That stated, it is important to interpret the error code correctly since it can be the result of multiple conditions.

First, the sensor may be correctly reporting the mass of air being admitted to the engine, and the sensor error code may be alerting you to a condition where the MAF IS correctly reporting flow, but that flow is not within an acceptable range for the engine RPM, throttle angle, and temperature. That actual rate (mass) of intake air can be affected by vacuum, as well as a host of other variables and not due to any fault of the MAF.

Second, it is possible that the MAF is under-reporting the mass of air being admitted, resulting in a lean mixture. This can be caused by a failed sensor, dirty sensor elements, poor electrical connections, or air leakage into the engine downstream of the MAF (which of course is not the fault of the MAF).

As you apparently surmised, a restricted exhaust will result in a lower differential pressure between the exhaust and intake (less indicated vacuum). The cause of a restricted exhaust could be a damaged cat (melt-down) due to a lean condition, extremely rich condition, age, physical damage to the cat, or fuel contamination. Any lean condition can be a result of low fuel pressure, incorrect oxygen sensor input, clogging or poorly flowing injectors, or other control system problems. Each of those potential causes can have a host of root causes, such as fuel pump performance, vacuum leaks, inlet leaks, A.I.R. diverter leaks, weak ignition, electrical connection problems, and others.

In short, the cat converter may have to be replaced/bypassed in order to perform a complete diagnosis, including leak checks, fuel pressure, OČ readings, etc.
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Old 08-23-2015, 10:27 AM   #4  
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Re: MAF Question.

Thanks Vader. One thing I failed to mention which is probably very significant - the cat had physical damage when I bought the car. In fact a bolt was broken and the case was cracked. I replaced the bolt and ad-hoc repaired the cracked case with high temp JB Weld.

I am theorizing that the cat had a sudden catastrophic failure secondary to this earlier damage and the code 34 may not indicate a MAF failure. Up until this point, the car has run virtually perfectly.
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Old 08-23-2015, 01:04 PM   #5  
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Re: MAF Question.

First time I've had a chance to look at the carnage in the daylight. If the cat was clogged, it's not anymore. It blew out the intermediate pipe and muffler with cat shrapnel. I've never seen anything like it before.

BTW, if anyone was following my yellow IROC thread, this was an original GM muffler tailpipe assembly I located and restored for it a few years ago. Luckily I have a spare.

Pics to follow.

Last edited by chazman; 08-23-2015 at 02:05 PM.
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Old 08-23-2015, 09:51 PM   #6  
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Re: MAF Question.

Fortunately, Walker pipes are still available.
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