Howdy everyone,

Here is another P1171/P1174 thread for the week.

My car is a daily driver, and I also have an UltraGauge, so I can attest that things have been fine. Idles at around 650-670 RPMs, starts without issues, runs well, and is quiet overall. No hesitation in the engine. I consistently average 12.7 MPG due to lift, oversized tires, and a bumper.

Today I had a shop install longer stainless steel brake lines and a genuine LR 180F thermostat. On the way from the shop, I got P1174 code and when I checked the gauge P1171 was there too. Then "check engine" light went on. I cleared the codes with the gauge, but I'd like to know what, if anything, could be triggering these lean codes after the repair.

Chances of O2 sensors going bad at the same time, now, and without any warning are slim. The sensors are less than 2 years old. I am banking on some vacuum line issues or on the MAF. I have a crack around the air box housing outlet, so perhaps some dirt got into the MAF and it is misreading the incoming levels of O2. Anything else?

Thank you,
-mc

 

Mine was due do low fuel level. Filled tank, codes gone. I think I weakened my fuel pump by keeping the fuel tank less rhan 1/3 full for so long. Trying to thwart gas thieves.

 

The P1171 and P1174 lean codes are very commonly caused by vacuum leaks. A smoke test is a proven method to identify the sources of the leaks.

In other cases reported on this forum where there were no other obvious leaks the codes were eventually cured by changing the injector O-rings, which can leak unmetered air into the system.

I'd start with a smoke test. You can find guidance online to perform a smoke test using a cigar, but many shops have machines that can do it more effectively. Good luck

 

Do a good visual. Vac lines sometimes get knocked loose doing other mx.
......

 

Looked at the car this morning with my UltraGauge set to display fuel trim levels. When warmed up, long-term fuel trim was around 2 in both banks. Under load when driving on a highway, the long term fuel trim was around 1. Checked the trims at idle, 1,5K and 2,5K RPMs. No sign of overcompensation for the lean mix. At 1,5K RPM there was a slight spike to about 4 in both banks, but that should be well within the margin.

Check the vacuum with the water test--no issues around the intake. If there is a leak, it would be somewhere else.

I got lucky in a sense that P1171 came up while I was looking at the gauge. Before the alarm went off, the long term trim in both banks was 0.78 and the short trim was close under 1.

The investigation will continue.

 

Getting closer. After driving the car around, I noticed that short term fuel trim in both banks spikes when the following two conditions happen:

1. The car is going dowhill
2. I press the brakes rather firmly

I can reproduce this consistently, as the ramp I take off the highway points downhill. Right in the middle of the ramp, short term trim goes to about 25, then immediately drops to normal levels. The computer takes some time to process all of this, but by the time I enter my garage I got p1174 and p1171 ;-)

Any ideas where I should be looking for vacuum leaks?

 

Things point to the brake booster. Here is an easy test for those of you who have UltraGauge. Monitor short term fuel trim for both banks while pumping the brakes. If STFT goes up, you're sucking "new" air in.

 

Verdict:

1. Brake booster line has issue--the part that went into the intake manifold is loose and needs replacement
2. Two fuel injectors on each side have bad o-rings

To test for the vacuum leak in the brake booster, pump the brakes at idle while monitoring short term trim. I t should not go up. Definitely not to +25.

To test for the vacuum leaks around the o-rings, spray brake cleaning fluid and see if the engine's RPMs increase. I did not do that initially as I did not want to burn the car and the parking lot down. I tried to spray water to see if I could make the engine choke, but I did not succeed. The shop used the brake cleaning fluid to detect the leaks successfully.

 

use Enginecodemaster.com to debug Discovery 2 engine codes.

I put all the codes from the Rave book into enginecodemaster.

http://enginecodemaster.com

The site does not look like much. but it works.