Should I Replace the o2's? If not, ??
 

I am contemplating replacing the oxygen sensors in my truck to fix the minor spark knock I've been hearing.

For some info about the truck and it's symptoms, it is a 99 5.4 with 58xxx miles. The o2 sensors are original to the best of my knowledge. I hear light pinging when the truck is under load around 2000 RPM. I have occasionally heard it at other RPM ranges, but mostly when held around 2 grand for any time. For example: Leave my subdivision and head out to the main road. From a stop, I turn right on to a slight grade with a speed limit of 55. I'll accelerate to about 2500 RPM, truck will shift and go down to 2000RPM. While I'm still slowly gaining speed, the RPM level does not change much at all. Before I get up to speed, the engine will give off one or two light clatter noises. In order to hear it you must have the radio and A/C blower off. This is not the only situation in which the truck will ping, but under similar loads, mostly around 2k RPM. Several things to note:

Noise is only heard when engine is warmed up.

Octane may or may not make a difference. It is hard to tell. Initially when I switched from cheap 87 octane to Chevron premium, it was drastically reduced. I then went from Chevron premium to Chevron regular with seemingly no change.

Never any pinging at WOT

Things I have already done: cleaned IAC,throttle body, and EGR ports(no blockage). New plugs (OEM Motorcraft) and one bad coil was replaced. Fuel filter was also replaced. I have also run 8oz of seafoam throught the brake booster. I did the whole procedure right with wiating time, but still got no smoke show. Maybe a light wisp when I first floored it. I then dumped the remaining 8oz into the tank.

At this point, it seems I have done everything simple and cheap. It would be awfully expensive to start replacing things until the symptoms are gone, asthere are still so many things it could be. Would it be a reasonable next step to replace the o2's? If I replaced them all, thats 200 bucks!

Forgive me for the long post, I just would like some advice as what to do next. I have also posted similar topics before, but never with all the info I have here. If anyone has any idears, lets hear em. Heck, if there is someone in the DFW area that thinks they might be able to help diganose that'd be great too.

Thanks
Brian

I'm not sure I can give you a proper answer, but I'm in a similiar situation.

I'm getting a miss at idle, and getting back on the gas under load.
My 98 has 93k though and needs a tune up (plugs/wires/fuel filter/pcv valve) pretty bad.

I plan to replace just the front two o2 sensors at the same time since I figure they need it to some degree anyhow.

My advice would be the same to you.
Start with just the front two, since the back two (I hear) rarely go bad.

Good Luck

I would not immediately suspect the O2 sensors as the cause of the knock.

My suspiscion would be the EGR system or an intake vacuum leak. Either would be likely (but not always) to turn on the CEL.

If you decide to replace the O2s, replace only the two upstream sensors as they are the only ones that control fuel trim.. Make certain you reset the PCM so that your fuel trim tables are reset.

Steve

I was just poking around under there and found something puzzling. Right behind the battery, there is some hard plastic vaccuum lines that T-off. One side goes in through the firewall and I was able to trace the other side to the EGR valve. There is another tube of the same kind(hard plastic, looks just like vacuum line mentioned above) That is open at both ends. It is run from up top near the battery to the empty area down by the A/C line inside some factory-looking wire loom. It is grooped with several other wires, but starts nowhere and goes nowhere. What the heck is that for? I did unplug one of the vacuum hoses to inspect it (looked dryrotted) and it hissed, so it was still holding vacuum from the last time it was driven(30 minutes or so). Does that indidcate that it doesn't have a vacuum-related problem? I was looking for the octane shorting bar, but don't know if my 99 5.4 has one or what it looks like.

BTW, my truck runs perfect and smooth. No misses or stumbles. It does seem to idle a little low, though. When warm it idles at 600 in gear and 750 in neutral/park. I don't know if that is low or not. What is the normal idle speed?

I would suspect the fuel is poor. Try some different gas and see if you still get it. Get away from Chevron maybe. Shell or Mobil or some other type may solve your problems.

Good luck...

I dont think changing O2 sensors is going to help your pinging, but like SHO said, if you decide to change them, I would only change the front sensors.

The vacuum lines you are referring to are for the front differential. It uses a vacuum system to engage/disengage. Although, it is odd that they are not connected to anything. When standing at the front of your truck, look behind the battery on the firewall, there should be a group of solenoids (black plastic) that have some more of those vacuum lines coming out of them. Two of them should go down to your differential.
I've also heard these lines do something with the A/C system?

If you havent' done so yet, I would check and clean the egr ports in the throttle body. If they are somewhat clogged I believe it could cause the thing to ping without turning on the CEL until it clogs completely.

Just a thought.