Trouble code and drive ability troubleshooting

[Poncherelli]

Hi there, just bought a 99 LC and trying to sort out it’s minor-ish problems. The first I’m tackling is a driveability issue where it seems to stutter a bit at idle, never killing the engine but it noticeably drops in rpm from around 1000 to 400 then revs back up. Also noticing that there is a loss of power and some coughing when accelerating at highway speeds.

Pulled an error code of P0171 and thought I’d start with the easy fix of MAF sensor. Pulled the old one and noticed corrosion and replaced. Truck drove much better for about 4 days (250miles or so) and didn’t have any idle or power issues, but quickly threw another code that wasn’t present before — P0155. Then at about 250 miles after the MAF change, the old symptoms were back. I’m going to change the O2 sensor but wondering if anyone has any insight and can suggest other fixes incase these symptoms come back after the O2 change.

 

[white_lx]

If you only have P0171 and not P0174 (bank 2) then the issue is most likely from the O2 sensor.

If you have P0171 and P0174, i.e. bank 1 and 2, it could be a vacuum leak, fuel pressure, or injector contamination/blockage.

No misfires?

 

[Poncherelli]

Nonmisfires that I can hear or see with diag codes. Thing that confuses me is the that initial code coresponds to engine lean bank 1 sensor is 1, but now that MAF is changed, error code corresponds to heater on other side (bank 2).

 

[white_lx]

Can you look at the live stream data? See what the long term and short term fuel trims are for both banks. That may help to determine if the problem is on one bank only. If ST+LT fuel trims reduce at higher rpm's, that is an indication of a leak.

The failing of the oxygen sensor could be because of a long term lean condition and may not be the root cause.

 

[Poncherelli]

Thx. I’m not a skilled mechanic so I can look at live stream but wasn’t sure how to decipher the info. I will look and see what’s happening and post. I will say — prior to changing the MAF, at idle the short term trim was negative and then rose. It was reading higher on bank 1 then on bank 2. Now that MAF is changed it is near zero at idle (somewhere around 0.4-0.6) Then climbs accordingly as I accelerate.

Again I’ll post here once I look, but my recollection is both rise as the truck accelerates.

 

[white_lx]

Also, since you have a low idle rpm: is your throttle body clean?

 

[Poncherelli]

Isles a

 

[Poncherelli]

Idle is at about 700, and no haven’t cleaned throttle body but it was first on my list until error codes were thrown.

Looking at Long term trim, it does go down a bit when rpms go up, then starts to increase. Not sure what that means. Going to throw o2 sensor in this weekend and see what changes.

 

[Poncherelli]

Update:

Took by my mechanic who replaced the o2 sensor, and suggested new spark plugs to see if the issue was misfire related. After changing the issue is much worse. It now seems most definitely like a misfire. Happens around 1200 rom and 1800-2000rpm mostly and when idling. Idle doesn’t drop, but engine stutters clearly. As the car warms up it is still apparent but much less obvious.

So far driven truck 75 miles but no engine codes sent. Evap and catalyst monitor not yet reporting. Not sure what to think next, but for Hail Mary and cost sake, threw in a bottle of fuel injector cleaner.

Any suggestions would be much appreciated. As of now mechanic saying wait until engine code is thrown before replacing more parts.

 

[Spike555]

O2's will not cause rough running engine. Misfires cause rough running and O2 codes.
Rough running is caused by fouled spark plugs, bad plug wires or bad coils. These trucks are coil on plug so no bad plug wires.
That means coils.

A vacuum leak can cause rough running and high idle speed. So before replacing the coils check for vacuum leaks, you can do this yourself.
Buy a can of brake cleaner, start the engine and let it warm up. Then spray the brake cleaner all around the engine, anywhere there is a hose going to the intake plenum, that is the thing directly ontop of the engine that looks like tubing for a pipe organ.
If the engine rpm changes pay extra attention to that area and spray some more.
Once you have found that area look for cracks on the ends of the rubber vacuum lines. Any large cracks need to be dealt with, you can cut off the cracked end and still use that line, unless the crack is to long that is.

A vacuum leak causes this condition because it is letting in to much unmetered air, the O2's then read the engine as running to lean and the computer then adds more fuel to compensate and you get a high idle and rough running because fuel and air trims are way off. This fouls the plugs and causes O2 codes.

This may or may not be your problem, but to find out all it's going to cost you is a $3 can of brake cleaner and 15min's under the hood.

 

[Magikarp]

Check the PVC valve and the grommet to see if any leak/dark spot around the PVC area. My grommet was cracking and gave me the P0171. I changed both the PCV and the grommet to fix the problem. Just be careful not to drop the grommet inside when try to remove it.

 

[Mauser]

I was having the same problems. I replaced vacuum lines, tried smoke testing, and everything else I could without throwing parts at it. I finally decided to remove the throttle body to clean it. When I looked in the intake manifold it was filthy so I pulled it apart and cleaned it. I am pretty sure the intake manifold gaskets were bad because after I put it back together my fuel trims were back down and the miss gone.

I have found that if you put it in gear and hold the brake while giving it throttle if it misses worse and wants to die it is a bad vacuum leak.

Also replace the seals on the fuel cap, dipstick, and oil cap. Lots of vacuum lost there too.

 

[Poncherelli]

So that’s two posts suggesting vacuum leaks...I think I’ll try to tackle that instead of coil packs..although I have 2 on order.

What I don’t understand—after my mechanic changed all 8 spark plugs the car was almost undriveable. Stuttered at idle, and through 1000-2500k rpm then a little smoother at higher rpm. But after a day of driving to try to get the car to send a diag code, the issue is better.

 

[Poncherelli]

Not totally gone by any means, but just a slight mis mostly when idling.

 

[white_lx]

Encouraging results.

What I don’t understand—after my mechanic changed all 8 spark plugs the car was almost undriveable. Stuttered at idle, and through 1000-2500k rpm then a little smoother at higher rpm. But after a day of driving to try to get the car to send a diag code, the issue is better.

It may have 'learned' to work around bad spark plugs and or O2 sensors. When the parts where replaced, it took some time for the ECM to re-learn.

It probably still be a very good idea to clean throttle body and disconnect the battery while doing this so the ECM can re-learn.

 

[Chris325i]

So that’s two posts suggesting vacuum leaks...I think I’ll try to tackle that instead of coil packs..although I have 2 on order.

What I don’t understand—after my mechanic changed all 8 spark plugs the car was almost undriveable. Stuttered at idle, and through 1000-2500k rpm then a little smoother at higher rpm. But after a day of driving to try to get the car to send a diag code, the issue is better.

Apart from the coils, about the only other thing you have to remove to change plugs is that secondary airbox thing on the passenger side. it has several vacuum hoses on it, have you checked all are attached? Also if they are brittle the r&r can crack them.

 

[Spike555]

So that’s two posts suggesting vacuum leaks...I think I’ll try to tackle that instead of coil packs..although I have 2 on order.

What I don’t understand—after my mechanic changed all 8 spark plugs the car was almost undriveable. Stuttered at idle, and through 1000-2500k rpm then a little smoother at higher rpm. But after a day of driving to try to get the car to send a diag code, the issue is better.

To me that says spark plugs were not gapped correctly or are of the wrong heat range. It also says to me bad coils or coils not seated correctly.

But still start with the vacuum leak.

 

[Poncherelli]

Brake cleaner in hand tomorrow...will report back

 

[2001LC]

I've gone through bringing back air and fuel to as like new as I possibly can. i.e. Filters, cleaning MAF & T-Body, vacuum lines & PCV, intake & fuel injector gaskets & seals, spark plugs, fuel injectors clean & rebuilt with all yielding very good results. To the point everything but coils done. But still have a little more fuel smell while idling in shop than I like, a slight rough idle only I'd notice and acceleration not as smooth as I'd like. Replace all coils and bam! like new again!

I've also had a coil that is going bad but not yet throwing a code. They'll tend to do this when hot. I've found them, by connecting to tech stream with mini VCI while engine hot. Then while engine running put in D with foot on brake and raise RPM. This puts engine under a load. Then watch "cylinder misfire count". When replacing coils avoid buying those cheap ebay coils. They are most likely counterfeit/knockoffs and don't work well or last long.

 

[Poncherelli]

Brake cleaner in hand and sprayed around...not a sound or RPM change. Most lines looked good. One vacuum line looked cracked a the ends but the clamps were well past those cracks. Might look into changing that one, but it’s not looking line the source of my issues. Still no engine codes after about 200 miles of driving, so the OBD isn’t giving any clues.

RPM at idle is is still stuttering once or twice in my daily commute, have definite lack of power and some stuttering while driving.

Again, my confusion is that the issues were immediately worse right after the spark plug and o2 sensor change, then in a day or so better, now a few days later it’s better still. Jus hate driving the truck around while I know there is an issue. I don’t want to foul more plugs or o2 sensors and I’m betting the performance and fuel efficiency isnt what it should be.