Looking at a friends stock 86 corvette that has just under 100k on the clock for a bouncing idle condition between 6 and 700 rpm. It almost feels like it has a more aggressive cam in it than what was installed. It is an early production 86 with cast iron heads.
I unplugged the MAF and removed from the air intake and throttle body, the idle didn’t change but the car feels like a much different car with a lot more power.
I reinstalled his MAF and swapped my MAF in and it went back to the way it was running previous to removing it.
Does this make any sense? I am scratching my head.
Thanks for your advice

 

This behavior is in no way conclusive of a bad maf sensor.

If the maf is unplugged, the ecm should set a code 33.

If a maf related code is active, the maf signal will be replaced for control purposes by the default airflow signal. The default airflow signal is rather crude and is calculated from tps voltage, rpm, IAC position plus an offset. It is then limited by the max maf vs rpm table for good measure.

The resulting default airflow signal should be a reasonable approximation of the actual airflow for a stock L98. Since this signal is primarily driven by the tps sensor, it will respond a bit quicker than the maf itself, since there is some delay between tps movement and a (measured as well as actual) change in airflow.

There are at least two possibilities:

1. The default airflow signal is somehow more accurate than the actual maf signal (calibration issue, or malfunctioning sensor).
2. Significant airflow is bypassing a properly functioning maf sensor with the maf connected resulting in a lean condition without adversely affecting the default airflow signal calculation while the maf is disconnected.

If the engine is surging for some other reasons, it is possible that the default airflow calculation is less sensitive to the surge resulting in more stable timing due to reduced load variation than would occur with a properly functioning maf.

 

Here's a possible theory based on reason #1 -- presented by tequilaboy.

O2 sensors are only present in one cylinder bank -- the DS. If 1 (or more) injectors are failing on that side, the sensor would be seeing a lean condition and add fuel. At some point, the other injectors will be pushed too rich -- affecting performance and idle. When the MAF is unplugged, it goes into "fault mode" and uses stock tables to fuel the car (versus the increased fuel "trims" adjusted as you drive). That would restore normal fueling.

I'm not a mechanic. I play one on TV! So...take this theory with the weight of it's source!

I'd OHM the injectors to see if one/more is failing. If so and it/they are on the DS, you found the problem.

 

Thanks for the help

 

Point of clarification: The long-term (BLM) and short-term (INT) fuel trims will still be active and compensate for mixture error (within limits) based upon O2 feedback independent of whether the maf sensor signal or the default airflow signal is in use.

Given enough time in closed loop mode for reasonable correction to occur across the relevant BLM cells, the resulting air/fuel mixture will be essentially the same with or without the maf sensor in the loop. Transient behavior may be a bit different due to the response time differences already mentioned.

Some idle variation is normal due to the controller deadband and limited effectiveness of the IAC in terms of response time.

Issues like this are very difficult to diagnose without scan data. Suggest you record lots of data with and without the maf sensor and look for trends in the fuel trim behavior for each case and compare the two signals directly under similar test conditions to identify differences in the calibration/calculation.

Note: 1986 cars (specifically with $32 masked bins) can be a challenge to connect at 8192 baud. For this reason, I suggest that you try DataCat first.

Comments regarding idle speed: 600-700 rpm is a bit high for an auto trans car in gear (550-575 rpm warm target idle is typical). You didn't mention if the car was an auto or manual and whether or not the idle behavior was in gear or park/neutral, nor the coolant temperature.

An auto trans car in park/neutral should idle around 600-625 rpm due to the 50 rpm park/neutral offset. Considering the control deadband and offset, 600-700 rpm sounds about right for an auto trans car in park/neutral, or a 4+3 car (w-700 rpm target idle).

The ecm can't really do much to dampen an oscillating idle if it should occur. The IAC is too slow, and spark advance is not used directly as a means of idle speed control.

 

Expanding on tequilaboy's input. I've found the majority of times a "hunting idle" is because folks have changed something like: timing (many times without disconnecting the wiring), TPS setting, or idle screw, without going through the procedures for "resetting" the minimal idle speed. The next most common is a dirty throttle body, particularly around the IAC seat.

If the inside of the throttle body doesn't look bad, I'd clean it on the car, pull the IAC, clean it and its seat at the throttle body, and go through the minimal idle procedures - which to do it "right" includes checking/setting the timing and the TPS. If you have to change much regarding any of the settings, it might take a couple "rinse and repeats" until you end up not having to touch anything. If the throttle body looks a little on the nasty side, I'd pull and soak it for a proper cleaning.

 

Your entire post has good stuff! I didn't know BLM/INT were still used in fault mode. I assume they disappear when battery is disconnected....meaning the results I suggested are possible if power was cut between MAF units?

FWIW...I don't really have hunting idle like I used to. During the past year, I replaced MAF, cleaned grounding post behind battery, and put in a new O2 sensor. (Prior MAF was de-screened...which might be the biggest reason?) Still have both MAFs. Been meaning to try descreened unit again.