Rockhound7

I recently had new spark plugs installed in my 2004 RX 330 by my dealer, along with some other maintenance recommended around the 125K mark. Since this happened, I've noticed what is likely a 2-2.5 mpg drop in my mileage, which is infuriating. It doesn't seem to be related to seasonal weather conditions/temperature. I used to get around 22 mpg in roughly a 50/50 combination of city/highway driving, and now I'm getting 19.5/20--tops. My tire pressure is fine, my driving habits are the same they were previously, I'm using the same gas, etc.., so I can't think of what's going on.

The problem is that I talked with the Lexus dealer about it, and they say they can't find anything wrong. No error codes, the engine runs smoothly, etc. I'm pretty frustrated because it is a noticeable drop (and as I figure it about a 10% increase in my fuel costs) but I'm not sure how to proceed at this point.

Do any of you have any ideas as to what could be going on, and/or other things I could have them check? Any help/ideas from you folks (especially any of you Lexus mechanics) would be GREATLY appreciated!

Larry

RX330inFL

Do you know what brand was used? Part number?

I believe NGK to be the best choice, however, others may have a different opinion. Other plug vendors can be used, such as Denso, with varying luck.

If the wrong or different heat range plug was installed that could make a difference even if NGK.

stunt66

I was going to suggest wrong plugs too but if it was done by a lexus dealer you'd figure they know what to use.

filmteknik

I don't suppose the service work just happened to coincide with the change to summer gas. I got in the habit of doing a mileage check with almost ever fill-up and the first time I saw a roughly 2 mpg drop I was wondering what was going on and checking tire pressure etc. I eventually realized it was the change in gas formulation in summer (lower volatility). In fall it went back up.

I know you said you are using the same gas but I presume that to mean brand & type. I don't know if you have taken the winter / summer change into account or perhaps they don't do this where you live.

Editing to add:
Googling around I came across an article discussing winter vs. summer and they were saying they got better mileage in summer. Either my experience was opposite or I am totally misremembering my own experience. Hmm...must ponder.

This is the perfect post to end with: YMMV

lex_rx

Gas pumps usually have a notice that says something like "pump is corrected to 15 degrees celsius" (about 59 F). A "gallon" of gas on a cool day is not necessarily the same "gallon" on a hot day. You may simply be putting "less" gas after the plug change.

If I can help it, I try to fill up either earlier in the morning or later in the evening.

RX330inFL

Agreed, but some will and do use Denso. Just like tires, there is more than one OEM.

rx400h1976

Mine is same as your experience - better mileage in Winter and 2 mpg drop in Summer (so far)

Lexmex

One thing also to keep in mind is that the vehicle was running with those old plugs for a while. After I change my spark plugs, I always disconnect the battery to reset the vehicle's computer. Otherwise, the vehicle is running on the assumption that nothing has changed since you last turned on the vehicle, despite having changed the spark plugs. This also goes for any time I think with the intake, exhaust or doing any kind of cleaning in the engine.

Rockhound7

Thanks for the responses, guys. I'm pretty sure they put Denso plugs in as replacements, so here are my questions:

1) What does Lexus put in from the factory? I'm assuming NGKs, but if one of you Lexus folks could tell me I'd really appreciate it. It's a 2004 RX 330.

2) Is there a difference between Denso and NGKs? I don't hear anything bad about NGKs, but I hear some yay and some nay about Densos. If Densos are used, do they need to be gapped differently than the NGKs? Again, I'd normally assume the Lexus dealer would know this but at this point I'm not quite so sure.

Part of the reason I've had it serviced at the dealer is that I expect them to use the same replacement parts as what was originally installed. I'm going to take it in on Thursday and they'll put a plug to see what's there. If they're Densos and it came with NGKs, I'm going to ask them to put NGKs back in. Is that reasonable? And if they won't do it, I'll take it to a Toyota dealer, have them do it, then call Lexus corporate and ask them why I have to take my Lexus to a Toyota dealer to get the same parts as replacements as those that came with it.

RX330inFL

http://www.ngksparkplugs.com/part_fi...engineid=30177

HtownBlue

Hound, and easy way to tell if a plug needs to be gapped is to look for a carboard sleeve around the business end of the plug. If it has one, there is no need to gap a plug. If not, chances are you need to gap it. MOST PLUGS, not all, come pre gapped now a days from the factory so no need to worry about that. Denso and NGK plugs all come pre gapped from factory.

As for your drop in mileage, you computer has to catch up and go through several engine cycles (off and on and mileage requirements) to make minute adjustments. Many people do not know that computers do not give codes immediately for misfires etc. until it is detected multiple times and under multiple cycles.
Your mileage drop I would bet comes from 3 potential causes (minus above). Possible failing coil. Coils can grow weak and not completely cause misfire. Second, your air intake system can be dirty. Clean the throttle body and air filter. Lastly, you could have a plug that was not completely tightened down. Now plugs shouldn't be tightened down past torque spec, but you would be surprised how many times mechanics don't completely screw in a plug during a change. Your spark is barely reaching the air fuel mix if this is the case.

RX330inFL

The best way to tell which plugs they used in your RX would be to pull one and look. However, you do not know what was there previous unless you had the ones they pulled. They could have been Denso, they could have been NGK. Just like tires... you could get Goodyear or you could get Michelin when the vehicle was new on the lot. Parts like this are multi-sourced. From the factory they may ALL be Denso. To be honest, I do not know that answer.

Did they replace NGK with Denso? Did they replace Denso with Denso? Did they replace NGK with NGK? Did they replace Denso with NGK?

You may get the dealer to pull a plug or an invoice to see what they put in, however, they will probably not be easily convinced to replace all if you are not happy with what they used. Also, there is nothing to say that when you go to a Toyota shop that they are not also using Denso as their primary.

Regarding your second question as to which plug is best, that is a difficult point to provide statistical evidence without extensive testing. Something I do not have. Here is my opinion. In a very generalized statement, I would say:

Chrysler vehicles run best on MOPAR
Ford vehicles run best on Autolite
GM vehicles run best on ACDelco
Japanese vehicles run best on NGK
German vehicles run best on BOSCH

Of course, there are always exceptions to the rule and some cars will run better of different plugs. That does not even take into consideration sub-models in each spark plug manufacturer's product line. You also have one, two, three and four electrode plugs. Note all the NGK plugs listed as replacements in my previous post.

Now, are you thoroughly confused yet?

The manual states:
"Your engine is fitted with iridium-tipped spark plugs."

So, if I was going out and purchasing new plugs for my RX I would go with NGK Iridium IX BKR6EIX-11.

YMMV.

Will try to get around to pulling a front plug tomorrow if possible to see what I have in my 2004 RX330.

salimshah

I am going on a limb, but Lexus would only put in recommended parts, or tell you to come back when the parts are in.

Why the focus on just the plugs? What else was done? Do you feel a difference in the roll?

My local gas stations started to post 10% ethanol sticker (and gas was modified) and my mpg has dropped.

Salim

RX330inFL

1. I agree that the focus on plugs may be a false lead. Was just going on details provided. If the original plugs were replaced same-with-same then this line of inquiry can be dropped and we can move on to other possible reasons for the problem. Am still curious to know which plugs, brand and part number, they did put in his RX.

2. I am assuming the OP has had their RX for many years and the noted drop in mileage is not something noted compared to just recently. Rather, this is also a comparison between summers and over years.

Here in FL the 10% ethanol stickers are up year-round. Do not think I can go to the normal stops and get fuel without ethanol. Therefore, my need to purchase marine fuel for my other vehicles. We do have fuel now being used which has been blended for summer use. However, I do not see much of a drop in my mileage. Believe my last tank used for just running around locally was 20MPG. Same as every other time of year.

mandyfig

Lexmex has a good suggestion, unplug the battery and let it re-learn.