avalon42

iTrader: (1)

...need advice from the almighty CL gods.

I have a 92 LS400, about 202K on the clock. Lately, I've been noticing on a full tank I've been getting around 250 miles. Before, it used to be around 400+ miles. I don't drive with a heavy foot because I don't want to pay speeding tickets anymore.

There is no check engine light, and yes I've checked for stored codes. Start-up in the morning show rich fuel vapor, and improves slightly when the car is warmed. I did change the plugs, wires, caps, and rotors. The old plugs look fouled. I did not check them recently, but I assume they might be the same. The rear of my car has that black soot like particles that accumulates from a vehicle running rich.

Two nagging things on my mind: both ignition coil packs or possibly the FPR (fuel pressure regulator). Please chip in because I feel there might be something I'm overlooking or should check when I tear into it tomorrow.

Once again, it throws no code whatsoever. A friend of mine claimed that one of my pistons may be losing compression but there is no noticeable loss in power so I don't know if I'd go down that route.

Thanks guys.

PureDrifter

iTrader: (10)

check the O2 sensor voltages

avalon42

iTrader: (1)

Think it might not be dropping in variance fast enough?

PureDrifter

iTrader: (10)

o2 sensors by design will die slowly over time, causing crappy performance and gas mileage wayyy before they trip the O2 sensor. search for the testing method.

avalon42

iTrader: (1)

I remember how do it, but I didn't think it would be that bad not to trip the CEL.

I'll double-check tomorrow for sure though.

Thermactor

iTrader: (1)

I'm interested in the outcome of your investigation - keep us posted!
My personal best on a tank was 520 miles before the fuel light came on.

Spacepig

I've noticed a recent 10+ % drop in mpg in my '98, which seemed to coincide with new tires (stock size Yoko V4S) in November, although it did not make sense that replacing stock tires with the same size could make that much difference in the MPG. I'm now wondering if is a result of California winter gas. I'm not sure exactly when they roll it out, and it may vary by location (I'm in the Bay Area), but I notice that the OP (and everyone in this thread) is in CA, so maybe the reformulated gas is part of the issue.

avalon42

iTrader: (1)

Ok, I did check the coils. Turns out the driver side coil was going out. Replaced and power has been restored, and the excessive fuel consumption/loss has been curbed--mostly. Tips on spark plugs were not fouled.

I did test the O2 sensors for resistance, and they were within range. However, I was wondering if anyone had noticed significantly better mpgs with brand new O2 sensors. I am sure mine are original. Some website made a claim that a 40% increase would be realized. If I get even a 10% efficiency, I'd go for it. Been eyeballing the Walker replacements.

I was also going to re-test my Engine Coolant Temp sensor as well, seeing how it has to do with fuel economy as well. I just replaced it with the Autozone one, but going back on some of the threads, I noticed some were complaining about the sensor going **** up before its time.

Any more input or insight would be appreciated.

Thermactor

iTrader: (1)

How did you make the determination that the driver's side coil was bad?
Replacing coils is fairly cheap and easy. I might want to do something similar with my sled. I'm sure the quarter million mile originals are probably tired.

avalon42

iTrader: (1)

Well, when my friend helped replace the starter, he accidentally cracked the connector. That, coupled with leaking oil from my valve covers, finally killed it. I thought it might be something ignition because I used to have a hesitation while waiting at a stop light. When I replaced the coils, the hesitation disappeared.

There is an actual test for resistance but looking at how it was already cracked and gooey, I didn't test the coils.

My odometer just past 204K so they served their time. O2 sensors are next on the list.

justinp95

You should go ahead and sea foam it too while your at it.