Running rich/poor MPG

[NEC]

Since I built my van 1 yr ago it has consistently got around 16-17mpg. All my ignition parts are new OEM quality and my timing is dead on. I recently started monitoring my oxygen sensor voltage to get an idea what's going on. At idle the voltage fluctuates as it should. During steady cruising on a 40 mile test drive the oxyen sensor voltage stayed steady between .6 and .9 volts, usually leaning toward the higher end of the scope. It did not fluctuate at cruising speed. This indicates a rich mixture. My new catalytic converter is already turning psychedelic colors from all the heat. I checked the voltage for the temp sensor at around .5v when warm. This means my temp sensor is at least working ok. I have not checked my airflow meter but I am tempted to remove the plastic cover and tinker with the spring tension. Anyone have any ideas on curing an over-rich condition?

[llamavan]

When there's a detectable component problem, the ECU defaults to rich, so that's where I'd start hunting (since you already checked timing and O2 sensor) — things that could cause the ECU to default to rich. Have you pulled codes? Have you replaced or swapped out the TPS (which can go bad enough to run rich without throwing a code)?

Gwen

[NEC]

My MIL is not lit up, but it does work during bulb test and timing check mode. I haven't pulled codes as I assume I don't have any. My TPS has been replaced with a verified good condition used unit and dialed in properly using a meter.

I ruled out a leaking cold start injector as that would show up at idle. It seems I can get the o2 signal to "bounce" during idle and up to around 2000 rpm in neutral. Cruising at any speed causes a steady needle on my meter, and rich on fuel. I know the computer is supposed to bounce the fuel trim rich/lean during cruising to keep the converter happy. The computer overrides this during heavy load, acceleration, and high rpms. My thought process is that my air flow sensor is telling the compy I'm pushing it a little harder than I really am, causing it to go rich to prevent backfire.

I took apart my MAF and ohmed it out. Everything checks out. I tightened the spring a few clicks and will hopefully get another test drive on it soon.

[llamavan]

The CEL won't necessarily light up if you have codes stored; in fact, IME, most of the times that there are codes, the CEL is not lit up (I've only had it go on — intermittently — for the O2 sensor and — steadily — for a completely non-functional thermo sensor). I even have someone else's van here for, among other things, a really trashed TPS. Actually throws the TPS code (unlike my '84 LE) ... but no CEL while running, ever ... not even when stumbling and stalling out. So my advice would be to see if you have any stored codes before assuming you don't.

Gwen

[NEC]

Well I checked for codes and got nothing but I also had not driven much since clearing the ECM. I will drive it a bit more and check again.

[pastadog]

Keep us posted!

[timsrv]

My TPS has been replaced with a verified good condition used unit and dialed in properly using a meter.

That's great that it's been properly dialed in, but a used TPS? Do you know history? I've seen so many bad TPS's that I often change them as preventive maintenance without even testing. IMO, these things are unreliable after 100k miles and should be replaced no matter how they test. Another thing to consider is a bad TPS can mess with the electronic advance resulting in excessive EGT's (Exhaust Gas Temperatures).

I'm not saying it's your TPS, but it's history sounds questionable and bad TPS's have been known to cause excessive EGT's, poor economy, and poor performance issues. And no, a bad or intermittent TPS will not necessarily cause your ECU to throw a code. I'll be the 1st to admit that changing your TPS may not change anything, but they are not that expensive and not that hard to change. Good luck. Tim

PS: I'd only mess with your MAF as a last resort. I'm sure you already know this, but don't do anything to it that can't be undone. Tim

BTW, I was in Yuba City for the last few days. I just got home tonight. I thought about bugging you, but I was busy with family .

[Flounder]

I'll be the 1st to admit that changing your TPS may not change anything, but they are not that expensive and not that hard to change. Good luck. Tim

Hi Tim,
Would you happen to have any hints or tips on replacing the TPS? I may have to do so on one of my vans soon. Thanks very much.
Tim

[timsrv]

I've talked about it a few times. The most thorough write-up is in THIS THREAD. They can be tested while installed, but the throttle body needs to come off for replacement (you can't get to the lower mount/adjustment screw with the throttle body installed). Tim

[NEC]

That's great that it's been properly dialed in, but a used TPS? Do you know history? I've seen so many bad TPS's that I often change them as preventive maintenance without even testing. IMO, these things are unreliable after 100k miles and should be replaced no matter how they test. Another thing to consider is a bad TPS can mess with the electronic advance resulting in excessive EGT's.

I got my TPS from a 91 Celica at the local yard. I took the cover off and inspected it. the plastic parts that fall apart/wear out were like new and all the contact points were clean and working properly.

You wouldn't have seen me this weekend as I was at Point Reyes Seashore and just got back in but you are welcome to bug me any time you are in the neighborhood.

I just picked up a new ECM and AFM from a very well maintained van at Sacramento Pick N Pull. We'll see how those work out soon.

[trestlehed]

NEC,

Any update on your TPS/ECU problems?