SES light came on today

[rockfangd]

Hi all. Unfortunately my light came on today. 1997 Cadillac D'elegance Deville

I dont idle my vehicles for anymore than a few minutes. I let them run for about 2 minutes and then take off.

Anyway I stopped to my bank and went through drive through and while sitting the light just came on.

I pulled 2 codes

P0171 current

P0174 History

Car seems to run good to me. I drive it hard daily and mainly highway speeds.

I know it has something to do with banks being lean from too much air or not enough fuel.

I dont have alot of time to look at it likely til spring, I have not cleared the codes. I figure it should clear by tomorrow if it doesnt see the problem again.

I know the 2000-05 has the coupler problem between the intake and TB but what about this generation.

Any suggestions very appreciated as always.

Temps will be very cold so I probably wont even open the hood til at least next weekend.

[Cadillac Jim]

P0171 Fuel Trim System Lean Bank 1 [CURRENT]
P0174 Fuel Trim System Lean Bank 2

Well, as you obviously are aware, you probably have a vacuum leak.

I looked at the "Throttle Body Assembly Replacement" section of the 1997 FSM, pages 6-712 and 713. The throttle body spacer looks like a solid piece, not rubber as in some later models. The EGR pipe is held to the spacer by a bolt with torque spec 21 lb-ft, implying that it is metal.

The FSM section on DTC P0171, pages 6-384 through 396, suggest these main points:

• If you have other DTCs, fix those problems first. I'm sure that doesn't include P0174, which will be almost identical.
• Check the air cleaner element, the air duct, and that the MAF is clear. Most people who replace air cleaner elements don't properly seat the cover, which has the IAT and intake air duct on it. I took mine out and used my body weight to get mine to close over a new air cleaner element when it got old.
• Check vacuum hoses and their connections.
• Check the throttle body, intake manifold, and EGR valve for leaks.
• Check the PCV valve and system for leaks, including a loose or missing oil filler cap or dipstick.
• Contaminated fuel (!!!) can cause this code. Elsewhere in the FSM it says that max ethanol content is 15%. Of course, water in the gas can clog the fuel filter or possibly the injectors and throw a P0171/P0174.
• Check the PCM and sensor *grounds* and make sure that they are to the right places.

I think all this is motherhood to someone with your experience but this is the FSM checklist.

If you have a good code reader, the PCM stores a freeze frame when this code is thrown. A CURRENT P0171 code guarantees that the freeze frame data is there, particularly if there is no other CURRENT code.

[rockfangd]

Thank you very much. I was exhausted when I posted this thread last night.

There is more to this.

I never run ethanol gas in any of my vehicles but ran into a situation where I filled my tank with 93 octane and it had ethanol. I got the car washed at the same time. halfway through the tank is when the light triggered. Maybe coincidence, but who knows.

I may try to change the oil tonight and will give it a good look over.

I have a trip to take sunday with it and dont want to run into any issues.

Also will be refueling tomorrow.

Hopefully it will go out.

If it doesnt I will hook up my scantool and check the freeze frame info. Should tell me a lot more.

[BodybyFisher]

This is a lean mixture that is affecting both banks. It can be a vacuum leak, however keep in mind that it also could be related to low fuel pressure also as that also can cause a lean mixture. But your statement that the car runs fine would seem to negate that possibility. But check fuel pressure.

Check vacuum hoses, brake booster vacuum hose, the intake manifold backfire valve, throttle body seal, pvc system,

[rockfangd]

Thank you for your response.

Just as I figured the light went out this afternoon. I actually knew when it went out because I felt a instant change in performance.

I will check the fuel pressure.

I changed the oil tonight with delco filter and rotella 10w30. Found nothing obvious wrong, no noises, etc...

It is back to being its rocket self lol. Big car with big balls.

Sadly still miss my 96. But still love it

[Cadillac Jim]

If the problem came and went, it may be water in the gas.

It's hard to avoid 10% alcohol in the gas. The number of stations that sell it is vanishingly small. I think that there are laws, possibly Federal, that mandate 10% ethanol. This was a big issue in the Ohio primary elections because of the farm lobby, and the large number of voters that benefit from higher corn prices (corn -> mash -> ethanol -> 10% of your gasoline). To find ethanol-free gas, see

http://pure-gas.org/

But, the 1997 FSM says that up to 15% is OK. I found that my gas mileage dropped about 1 mpg when they started using 10% ethanol.

Note that when you have 10% ethanol in the gas, it absorbs the moisture in the gas better, sort of like having STP Gas Treatment (with its methanol to absorb water) in the gas.

I recently read that part of the conversion of an engine to run E85 (up to 85% ethanol) is putting in bigger injectors because you need more alcohol volume to get the best mixture and make the HO2 sensors happy. So, with 10% ethanol in the gas, any issue that reduces gas flow will make it a little easier to make the PCM throw the lean codes.

[barczy01]

Make sure your intake manifold bolts are tight.

[rockfangd]

you know that is a good idea to check out.

I am curious as to how they ran the wiring to the starter and knock sensor.

If I check the old engine and see cut wires then I know they cut and spliced the wires from the replacement engine.

If the wires are not on the old engine then I know they pulled the intake and did not splice the wires.

Very likely chance though that they cut and spliced the wiring.

The light had come back on and went back out in the same tank.

It has not come back on in the next tank of gas which has 250 miles on it. Will have to see if it comes back

[BodybyFisher]

I hope not. The oil.pressure switch is on the same electrical conduit as; AC clutch, oil level, crank sensors, front O2 sensor and alternator field. The starter and knock sensor is a different harness

[rockfangd]

Corrected. Meant the knock and starter

[Cadillac Jim]

If you find a simple splice in the wiring where they cut the wires and used the old ones with the sensor connectors, replace the splice with a weatherproof one that you do yourself, and check the connector out too. There are knock sensor codes and such but I never trust anyone else's wiring.

The interesting thing to me is that the code, and full performance, come and go, like a switch is thrown. And, it's more consistent on the rear bank (Bank 1) when it happens.

I think that you are on the right track, looking at throttle body spacers and gaskets, intake manifold gaskets, and the usual PCV lines and other vacuum lines to the throttle body. It takes a pretty good vacuum leak to throw both codes and keep one CURRENT.

[BodybyFisher]

Corrected. Meant the knock and starter

As someone mentioned check the intake bolts, i just replaced a starter in a 98 that had its engine replaced last year and the intake bolts were loose

[rockfangd]

Well to update this, the problem was very likely to be bad gas. I among many other people had issues around the same time that were related to poor gas quality. After that tank I have had no problem

[Cadillac Jim]

Thank you for getting back to us. This is important to know when looking at similar problems in the future.

Note that contaminated fuel was on the FSM checklist for the DTCs listed.

[Phillip Hastings]

Taught so it was bad gas...

[Cadillac Jim]

If you were getting both lean bank codes P0171 and P0174, the gas was worse than poor quality, it had water in it, causing occasional injector or fuel filter clogging. This sometimes happens with stations that don't sell much gas, and let the tank go nearly empty. A ground tank that has lots of air in it will collect some condensed water under the gas. The pickup is usually on a floater to avoid this but when the tank is near empty, it will pick up some water if it is there.

[rockfangd]

well we had a bit of rain around that time. Being I only run premium maybe it is not sold all that much

[Cadillac Jim]

I found that recent rain didn't have much to do with condensation in underground tanks, it was how many months it had been since they were serviced and refilled. I don't know if they run a siphon hose down there and get the dregs out of the bottom of the tanks when they refill them, but bad gas is near-zero probability in busy stations so I suppose that something like that happens.

With the current Government requirement of 10% ethanol, condensation tends to be absorbed, within limits. That may have something to do with how rare bad gas is these days.

I once had a light miss that I thought was bad gas. It went away with a can of Techroline, but came back later. It turned out to be the plug wires breaking down.

[KHE]

If you were getting both lean bank codes P0171 and P0174, the gas was worse than poor quality, it had water in it, causing occasional injector or fuel filter clogging. This sometimes happens with stations that don't sell much gas, and let the tank go nearly empty. A ground tank that has lots of air in it will collect some condensed water under the gas. The pickup is usually on a floater to avoid this but when the tank is near empty, it will pick up some water if it is there.

No - Those codes in tandem mean there is a lean condition affecting both banks of cylinders. A vacuum leak is the prime suspect - A re-torque of the intake manifold bolts would be my first priority followed by a close inspection of the air intake piping.