AZjason

My wife daily drives a 99 4Runner. I replaced the spark plugs with the correct Toyota dual electrode plugs. I also replaced the air-filter with a new Toyota filter. Now her gas mileage seems bad. I have an OBDII scanner so I connected it, and it shows no codes. I thought a plug may not be firing. If the truck was misfiring, wouldn't the OBDII tell me?

The truck drives as I would expect, with no power loss.


BTW this is a crosspost from the 4runner.org forums.

gpcollen1

Well too small a gap can result in poor gas mileage, but not certain that is the issue, though related to the plug change.

91TPU

pull all the plugs one by one and check them...look for fouled plugs or plugs covered in fuel...and check to make sure everything is gapped correctly. also, check your spark plug wires, if they are old, maybe your gettin a bit of arcing, or they are absorbing moisture

AZjason

I don't think you can gap the dual electrode plugs? Either way, I do need to read the plugs and see if anything is obvious. It seems like if there were a misfire, the OBDII would catch it.

surf4runner

was the ECU reset after? it could take a full tank before it relearns from the changes.
try cleaning the MAF too.

kevin444

unplug the battery for 5mins replug it and drive around this should reset the ecu.

AZjason

I didn't reset the ECU, didn't even think about it. I'll try that tonight. That would make sense. The truck runs fine, no misses, scanner shows no codes so I think thats a really good tip.

sharrack

So it sounds like you had bad mileage BERFORE the tune up ...right??
Sounds like a classic O2 sensor needs replacing if your near 75,000
My mileage went south at 75,000 for no reason with no codes or bad running.
I could watch the guafe drop! Literally.
I just installed the #1 sensor to the front of the cat. 139.00!!!
There is a tech article out there on 4X4wire.com that explains this.

could be your deal...........Kenny S.
99 4runner 3.4 V6

AZjason

Actually, she noticed the mileage difference right after I did the plugs. However, the mileage is like 83k. So the O2 sensors are known to fail at this mileage? Are there 2 or 4 sensors?

nick-m

There are two 02 sensors, one in the front and one in the rear.

You should reset the ECU first, though...and worry about the 02 sensors a little after, should everything more or less return the way it was. They do tend to go bad after 75k and they do affect mileage, so it might not be a bad idea to replace them.

My mileage was looking REALLY awful 'till I replaced both 02 sensors. Now it's more or less OK.

AZjason

Absolutely, that doesn't cost anything!

MTL_4runner

Ditto on the above.....pull the neg batt cable, reset the ECU and drive to check mileage. If your mileage is still bad you can start looking at the MAF and front O2 sensor as possible culprits. Often the ECU just needs time to adjust and may run poorly until new fuel trims have been established (the ECU reset just accelerates this process).

How did the plugs look by the way? All same color?

AZjason

Yeah, they were all the same color. I don't think she has ever changed them either. They were pretty used up though. The center electrode had nice divots where the 2 outside electrodes are. Something that surprised me... One side of the engine had factory plugs, the other side had NGK's. All dual electrode, just different mfr. Like I said, I don't think she has ever changed the plugs and she is the original owner of the truck. Thats one of the reasons I was surprised at her fuel consumption. The old plugs were so tired, I thought for sure she would see better mileage. I'm unplugging the battery tonight. For what 5 or 10 minutes? Will she lose her radio stations?

MTL_4runner

Yep, those were definately the original plugs because the vehicles came from the factory with Denso pulgs in one cylinder bank and NGK in the other. Usually you can tell the plugs have been changed at least once when they are all the same brand (all Denso or all NGK). Sounds like the plugs were in really roough shape and the ECU was just compensating as best it could to keep the engine running.

Yes, you'll need to redo the stations. You can try pluuing the ECU fuse, but I prefer the batt cable since I have seen cases where the fuse didn't reset everything properly. Your call on that though.....about 5-10 min is more than enough.

BruceTS

Not true, all mine were Denso plugs from the factory.....

MTL_4runner

Well, obviously there may be an anomaly here and there (pehaps in manufacturing supply, perhaps they had been changed at the dealer prior to delivery, etc). I would still maintain that if you have Densos on one bank and NGKs on the other, you're probably dealing with the original plugs on the vehicle.

AZjason

I'm 99% sure these were original. She got the truck new in 1999, right before we met. When I pulled them I was surprised, so I asked her if they had ever been changed or if the truck had ever been in for some kind of recall. She said it hasn't so they must be OEM. It kind of made me nervous, I thought maybe something bad happened to this truck at the dealer before she bought it.

I wonder if Toyota stocks the heads at the factory with the plugs pre-loaded? When they pick the parts for assembly, you get whatever plugs were pre-loaded in the heads. Thats the only explanation I can come up with.

BTW I reset the ECU last night, so hopefully that cures the MPG problem, Thanks for the tips.

nick-m

Well, I wouldn't go as far as saying that isn't true; I had the same exact thing and I've heard of others having a 50/50 of Denso/NGK plugs from the factory...for instance, my '96 had Denso plugs on one side and NGK on the other. They were original...pretty much everything was original, actually, when I bought it....10 years after it was made

91TPU

any particular reason they did denso in one side and ngk in the other? sounds like some sort of comparison test or something

MTL_4runner

There has been alot of speculation as to why and many threads on it, but I don't think the actual rationale was ever confirmed by someone working for Toyota manufacturing (and since most of these vehicles in the late 1990's were made in Japan, that might be a bit hard to do). If someone can verify one way or another, I'm all ears.

Let's not hijack AZjason's thread too badly here though.