Mechanics

PCM P0171 & P0174 HISTORY CODES

2000 Cadillac Seville • 135,000 miles

I know these code are telling my car is running lean, but every I check my OBD, they come up "History". If I clear the codes, they light up my "check engine light". I read different threads about checking vacuum hoses, to be honest guys, there's so many different hoses under the hood, I don't know what's what. I read also, not to worry about it if it reads "history". Well, it keeps turning my light on and it's annoying. Any suggestions?
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Thunder
June 21, 2012.




The first thing you need to do is check for a vaccuum leak. You can use carb spray.

Saturntech9
Jun 21, 2012.
If you want to pursue the vacuum hoses, check at the auto parts stores that rent or borrow tools to see if they have a "smoke machine". That puts a white, non-toxic smoke into the system under two pounds of pressure, then you search for where it's seeping out. You can inject it into any vacuum hose or port.

A more common cause of fuel mixture problems that affects GM cars more than most other brands is mismatched fuel injectors. I only remember two brands from a very high-level school taught by an injector specialist. Chrysler buys their injectors from Bosch in flow-matched sets of four, six, or eight injectors. GM grabs a handful out of a huge bin and stuffs them in an engine with no regard to flow-matching them. By the time they get to around 100,000 and have some wear on them, a few will flow enough less fuel than the others that those cylinders are a little lean. The unburned oxygen is detected by the oxygen sensors and in response, the Engine Computer requests more fuel to all of the cylinders on that side of the engine. No matter how much extra fuel it requests, there will still be that unburned oxygen being detected, hence the lean fault code. Oxygen sensors don't detect unburned fuel, just oxygen. Rebuilt replacement injectors is the fix for that. They can only be purchased in matched sets. About 98 percent of that company's sales are for GM products. Most of the customers are amazed at how smoothly their engines run with those new injectors. That has been the solution to a lot of running and Check Engine light problems that have defied extensive diagnostic attempts.

Low fuel pressure will result in a lean condition too although that is a sign of a pump that may be failing and that won't typically go on for very long. The low pressure would cause a hesitation or stumble too.

Caradiodoc
Jun 21, 2012.
I wish the auto parts stores rented smoke machines lol. Get some carb spray let it idle spray around the intake gasket vacuum lines anywhere vacuum would be. If the engine stumbles rpms go up or down or dies there is a vacuum leak there. As far as the gm injectors I really havent seen the issues with the injectors. Getting dirty needing cleaning yes but going bad really havent seen that problem.

Saturntech9
Jun 21, 2012.
Thanks for the fast response guys. I got my fuel system cleaned about a yr ago, and my gas gauge (pump) hasn't been right since. I do have a slight hesitation & the car stumbles around 27 mph (or btw 2&3 gear). I thought it was a tranny problem. I have to put gas in it every week because I really don't know how much gas is in the tank(lol). Where is the intake gasket located? Does it look like any other corkboard ordinary gasket?

Tiny
Thunder
Jun 21, 2012.
Not that they're bad, in fact a lot of people obsess over the resistance checks and fail injectors that are still okay. It's just that if you put them in a fixture, side-by-side and watch how much fuel sprays into measured test tubes, they're all different. The story I shared is the story I got, but you have to consider carbon and varnish deposits that build up over time. Obviously those injectors performed satisfactorily when they were new. That could be why cleaning chemicals and machines are so popular and productive.

I hadn't thought about the carb spray. I was envisioning a complicated-looking maze of hoses and wires. I use water when the engine is still cold because I don't have to BUY water. The problem is you have to see or hear it get sucked in and often the leak is too small to be noticed. In that respect, carburetor cleaner is a better choice. You still might not be able to see where it's getting sucked in but at least it will get you real close so you have a smaller area to inspect.

Don't overlook leakage past the throttle shaft bushings. I've never found that myself but I have read about that being the solution a few times.

Caradiodoc
Jun 21, 2012.
To answer the question about where the intake gasket is follow where the throttle body is bolted too. That is the intake follow it down to the engine where all the bolts that hold it in place. Thats where you will find the intake gasket. As far as the injectors go gm has bulletins for them being clogged. The flow problem is hard to belive because gm uses a afid tester I know I spelled that wrong lol. It hooks right to the obd2 connector and the fuel pressure test port. It does a pretty comprehensive test it cycles the injectors thru the computer and measures fuel pressure drop to test the injectors also figures flow rate of each injector. If they were that mismatched it would make that tester results useless. Also pre that tester gm uses a injector balance test which you use a injector pulser. You watch fuel pressure drop while pulsing the injectors for the same amount of time. Watching for a difference from one injector to the other.

Saturntech9
Jun 21, 2012.
That's an interesting-sounding test, but what led to their need for it? Keep in mind too that the school I went to was about six years ago and at that time he was talking about cars old enough to have a history.

Is that test done with GM's scanner or does it require some other special equipment?

As for the service bulletin on clogged injectors, what was the recommendation? Could they be cleaned, as in varnish buildup, or did it have to do with a blocked screen? Could those be cleaned or did you have to replace the injectors?

Caradiodoc
Jun 21, 2012.
Honestly I only used the afid tester once by the request of tech assit hotline I did the afid test it came out fine. They wanted me to replace the injectors anyways and if that didnt fix to replace the ecm. It was a phatom random missfire it would start missfiring then suddenly stop. Very random I told my boss I dont see all the injectors failing all at once and that I thought it was the ecm. He said replace the ecm replaced the ecm car was fixed. This was the saturn aura which is a rebadged chevy malibu. The afid tester has a built in computer no need for the tech2 scan tool when using that. Its really expensive tester and a pain to set up and run all the tests it has. The older injector balance test doesnt use a scan tool ethier you unplug the injector harness from the injectors and plug the kent moore injector tester on each injector to pulse them. You can also use that tester to test transmission actuator/solenoids. As far as the gm clogged injector bulletin you use the otc pressure canister to run the car with cleaner plus put a bottle of chevron techroline cleaner in the gas tank.
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Saturntech9
Jun 21, 2012.
The afid tester is actually used to scope the injectors flow electrically etc a comprehensive test of the injectors while they are working.

Saturntech9
Jun 21, 2012.
Geez, something tells me I'm going to have to take it to the shop. $15 for the part & $500 for the labor. How long can I drive it in this condition?

Tiny
Thunder
Jun 22, 2012.

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