I have a 1998 F150 2x4 with a 4.6 V8. The motor knocks hard under power and lately has begun to stumble really bad under power. The engine codes report that both the FI banks are lean. The codes are 171 and 173 I think. Don't quote me on the codes I am going from memory, but I know they indicated a lean condition. I have relplaced the pre-cat O2 sensors and the mass air flow sensor. The situation improved some, but was intermittent this morning on my way into work. I recently changed the fuel filter too. I would appreciate any advice.

 

It was most likely a p0171 and p0174. You're right, they are lean codes, however there was nothing wrong with the o2 sensors. Check the elbow on the pcv hose behind the throttlebody I bet you it has a hole in it. The new hose should be in stock at your local dealer.

Robbie

 

Thanks for the reply. I checked the elbow and it was fine, but I went ahead and replaced it along with the PCV valve. The hose leading from the other valve cover was a little loose so I fixed all of that too. It seems to run a little better, but still stumbles alot when accelerating. I may have to bite the bullet and take it somewhere to have it fixed (which I hate). I checked all of the vacuum lines and everthing seemed to be fine. I am just afraid it will leave me on the side of the road or do some serious engine damage from running so lean. Let me know if you can think of anything else for me to try. Thanks!

 

Anybody else have any advice? Thanks.

 

Hey Nabbit, did your Check Engine Light come on? ...or did you just notice the problem?

My truck pings at high RPM also and now you've got me worried

How many miles do you have?

 

I did get a check engine light and the codes indicated that bank 1 and bank 2 are running lean. I have about 98,000 miles. This is turning into a huge pain in the butt for me.

Nabbit

 

Did you try cleaning fuel injectors?

 

I had an issue where I had to have some fuel injectors replaced about 30,000 miles ago. I have used the type of injection cleaner that you add to the gas, but that didn't help. I guess it could be a clogged or bad injector, but I don't think that would affect both banks. I guess I'll have to bite the bullet and take it in to have it looked at.

 

Whoa, let's talk about lean codes. Fueling is controlled by the ECT, IAT, and MAF only under two conditions, before the O2 sensors warm up (maybe 2-5minutes) and when at wide open throttle (WOT, usually defined in the PCM as >80% throttle). If you ONLY ping under either or both of those two conditions, the MAF could be bad. If you ping under partial throttle or when the truck is cold, it isn't the MAF. Another thing, unplug the MAF and see if it still does it. If it pings with or without the MAF, it isn't the MAF. The two temperatures, engine coolant and intake air would only make it rich at low temps. They never overlean so don't worry about them.

The rest of the time, after warmed up and not at WOT, fueling is controlled by the O2 sensors. If you ping at mid throttle but NOT at WOT, the O2 sensors are toast. (I know, maybe EGR overcame the pinging at WOT, but highly unlikely).

Lean (set codes and causes pinging) is probably neither of these things. Trucks usually ping or rattle starting at mid throttle and continuing through WOT. In this case, the fuel calculation is correct but something else is causing lean conditions. The usual suspects:

1. Intake leak. manifold gaskets, throttle body hinge, PCV valve or hose, vacuum hose off, brake booster hose, loose throttle body, etc. These leaks only affect you when vacuum is high and the air inducted is low. At mid throttle and WOT the vacuum is low and the amount of air would overwhelm a leak. This is NOT your problem. They would cause rough idle or high idle but be reduced by less vacuum and overwhelmed by higher air flows.

2. Fuel pump and fuel filter. This is the smart money. The demand on the pump and through the filter is highest under high loads (WOT and high RPMs). If the pump or filter is inadequate, it will be worse under high load. This is also when pinging is the worst. With a scan tool that will read PIDs, you can read the bank1s1 or bank2s1 on the highway, put it in a high gear at low speed and hold WOT for say 10 seconds. The O2 sensors should go high and stay there (WOT enrichment). If they go low instead, bet on your fuel pump or filter (I know, the MAF could still be involved. If you suspect it unplug it and try it again. Still low, you're damn sure). Or connect a fuel pressure gauge to the test port on the rail and bring it through a window. Same WOT test.

Screen around the pump could be the problem. If you replace the filter and still have the problem and think it's the pump, take it out before buying a new one. If screen is plugged, clean it first.

3. Plugged or dirty injectors. Less likely unless the lean is only on a few cylinders. You could check the plugs and see if only a few are bone white (the ceramic around the center electrode, down in the bowels of the plug is bone white) after a WOT run. Or you could try running injector cleaner and seeing if it helps.

4. Some injectors not commanded open. Use a stethoscope (sp?) and listen to each one (or use a long 3/8" extension). Again, only a few plugs would be white after long WOT.

If your truck just pinged or rattled, we would consider the EGR system, for carbon deposits, and maybe the timing. But since it set a lean code, we're pretty sure that's also the cause of the pinging.

Hope that helps. Again, this is why the code will NOT, probably EVER, identify the part that needs replaced. Only diagnostics will. If I were AutoZone, I'd provide free code readers too.

Mark Cantrell

 

Sorry for being windy, I just re-read the post and mentioned unplugging the MAF twice. The open loop management, cold and WOT, has to measure airflow to figure how much gas to inject. This is the MAF. There are two backup systems, one is called speed density using a MAP sensor to measure manifold pressure and the RPM. Knowing the displacement, the air density (pressure) in the manifold and the RPM, you can calculate the amount of air. Almost all GM cars have a MAP and use speed density for primary or backup airflow. My truck doesn't even have a MAP.

If there isn't a MAP sensor, the backup is Alpha-N. This uses the TPS and the RPM to calculate air. It isn't as precise as a MAF but is a useful backup. To activate it, unplug the MAF. In either case, MAF or Alpha-N, it is only used cold and WOT.

You can also make it stay in open loop and never use the O2 sensors for fueling. Just unplug one of the O2 sensors in front of the cats. Backup for closed loop is open loop, MAF or Alpha-N.

Again, sorry of being windy.

 

Turns out that it was a bad fuel pump. My fuel pressure was below 25# and I believe it should be 38 to 45#. I should have it back on the road by the weekend, I am just waiting for the fuel pump to arrive. I am having a shop do it just for the sake of time....I just don't have the time right now. I'll let y'all know how it runs when it's finished. Thanks for all of your replies!

 

I got the fuel pump replaced (pressure now 45#), but I had a bad spark plug wire that was arching. I replaced all of the plug wires and the truck test drove fine....it had great power and acceleration with no pinging. This morning I started it up and the damn thing wouldn't idle. I kept my foot on the gas for a bit and let the truck warm up. Then I shut it off and checked all the vacuum lines and plug wires, pushed everything on tight again. Then the truck started and ran fine. I am not sure If it was something I did under the hood or if it was the mere fact that I allowed the truck to warm up. The truck has been sitting for a couple of hours now, I'll go see if it will idle when it's cold when I have time (working now).

Any ideas?

 

Your comp. was probobly reseting all its paramitters and should be fine after about 50mi. of driving.

 

Truck runs better than it ever did now. It seems it was a combination of a plug wire and the low fuel pressure caused by the bad fuel pump. It hasn't run this good in years. All the power is back and no pinging on 87 octane!!!

Todd