[V8] Do bad/dirty sensors always throw codes?

Tony and Lillie tonyandlillie1 at earthlink.net
Wed Feb 18 13:19:49 PST 2009

I have a little btidbit of BTDT on mine.

First a background. I bought my V8Q (1991 3.6 auto) in 2003, with 253k on 
it. One of the things stated as wrong with it was that it took two or three 
times to start the car from cold. When I picked the car up, I didn't know 
how bad the problem really was, because it was already running.

After MUCH troubleshooting, I found the ECU was bad. I put a 1990 ECU from a 
parts car in there, and the car has started right up when cold from then on.

Now, when the car had the problem at first, I would occasionally use 
starting fluid to start the car. Well, when I say "I", I really mean Lillie, 
as it was her car. One morning, I got a call that the car wouldn't start, 
even with the ether. So, I came home,and it started right up. Well, what I 
didn't realize is that the car backfired when she was trying to start, and 
burned the pool of ether that was in the intake. In the process, it badly 
burned both the intake temp sensor, and the MAF. There were no indications 
that this had happened. However, over the next several months (years) milage 
slowly dropped from 25 hwy to about 21 hwy, and power was a bit down. As it 
got worse (eventually about 15mpg hwy) the car would billow black smoke and 
had a noticable power decrease. I had a 4-wire that had been replaced 
several times, and new caps/rotors/plugs. New FPR also. Well, long story 
short, I pulled it one day as a reference for a project, and noticed how 
burned it had become. Put another on there, and the problems I was having 
were gone.

In this time, I pulled codes at least 10 times, the only code I ever got was 
something along the lines of "Idle control rexceeded maximum specification". 
So, long story short, yes, sensors can go bad without throwing codes.

This is one significant difference between newer cars, and the pre-obd2 
cars. Now, they compare a # of variables, and if one component doesn't read 
correctly, it throws that code. On my car, only unplugging the MAF threw a 
MAF code, despite the fact that the readings were WAY off.

Tony Hoffman

----- Original Message ----- 
Subject: [V8] Do bad/dirty sensors always throw codes?

> On V8 #2, my starting point was a car that got 10mpg, was as loud as a 
> Harley, little power on acceleration, and it just feels like the engine is 
> missing.
> After working on the exhaust (replaced clogged cats and rusty exhaust 
> pipe), swapping out the FPR, and disconnecting the OXS, among other 
> things, the car now gets 14.3mpg in town and has more power, but it's 
> still not right. Still lopes a little and feels like it's missing on 
> acceleration, though appears to run smoothly once up to speed.
> Only code I've ever gotten is 2341. After disconnecting the OXS, I get 
> 2342 (which sounds like what I should expect with the OXS disconnected).
> Other folks with similar performance problems have replaced/cleaned the 
> MAF and coolant temp sensors, according to the archives. However, I'm not 
> getting codes for either of these. Is it possible that the sensors are 
> contributing to the problem, but not enough to result in a code?

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