Drop In Mpg After Exhaust Repair

[adrian torres]

Help!,

My 1997 LS400 has had a drop in MPG from around 20mpg to 15mpg after my Lexus was serviced at the local dealership. I had to replace a damaged driver side heat shield that was dented. The dealership had to "drop" the exhaust in order to replace the shield. After that repair, my mpg had dropped and never recovered. I took it back to them, and they could find nothing wrong. I tried to drive during different times of day, commute routes, etc. and they are all result in the same low mpg. However, highway mileage is stayed at 25mpg when I drive on the interstate at a constant 65 mph.

I mentioned to them the water temp sensor that everyone in the forums point to, and the dealership did not think that it was the cause, since the drop occured after they serviced it. They also stated that the exhaust drop would have nothing to with a drop in mpg. They tested the o2 sensors and they were working fine, and there is no engine light. They stated that the car was fine, however they could replace the sensor for $250 but would not recommend it as it would not solve the problem.

I wonder if the rear o2 sensor is sensing a air leak, if the dropping the exhaust caused a seal to break and this is causing the car to run rich.

I am running out of ideas and the drop is causing my fuel costs to increase 25%.

Any ideas or suggestions?

Adrian

[landar]

I would put a scanner on it and look at the fuel trim. Should show that it is running rich. How many miles on your '97?

[curiousB]

The secondary O2 sensors merely exist to check the performance of the catalytic converter. The first O2 sensors are the ones used to actively trim the fuel mixture to achieve optimal fuel usage. I doubt any of this has a bearing on fuel efficiency however.

I'd suggest:

1. As Landar suggests take a look at short and long term fuel trim. This can be an indiaction of a problem. If you have a laptop based OBDII unit look at O2 sensor graphical outputs.
2. How sure are you that it was better before and isn't now? MPG needs to be computed over a full tank and over equivelent conditions to allow comparisons. A lot of people cite MPG concerns but in the end they lack the data to back it up. Do the math.
3. Are you tires at proper inflation. This can kill your MPG.
4. Are the brakes dragging? Maybe a sticky caliper is dragging you down... Use a non contact ir temperature gun to measure brake rotors after a short drive. They should all be approximately the same temperature. If one is hotter then maybe it is dragging and caliper is sticking.
5. Dirty air filter?

[fsuguy]

Hi,

This is not to detract from the sage advice given already by engineers and the very experienced - check those suggestions out as well; however, under the assumption that you are pretty confident about the state of your car/performance before the work done by the shop, and other things being equal, let me suggest there is a good possibility that the O2 sensors actually may have been affected since the work was on the exhaust system. The following is extracted directly from a Toyota manual on ECU performance (note the references to injector duration):

Engine Duration Correcton Factors

"...The second step involves duration corrections. Input sensors used for Injection duration corrections are:

1. Engine Water Temperature (THW)

2. Intake Air Temperature (THA)

3. Throttle Angle (VTA or IDL & PSW)

4. Exhaust Oxygen Content (OX)

Once basic injection duration is calculated the ECU must modify the injection duration based on other changing variables. Variables considered in the correction calculations are coolant and intake air temperature, throttle position and exhaust oxygen sensor feedback (when operating in closed loop).

* As engine and intake air temperatures move from cold to warm, injection duration is reduced.

* As the throttle opens (IDL contact break), injection frequency is momentarily increased.

* Fuel injection duration swings back and forth between longer and shorter durations to correct conditions detected by the

exhaust oxygen sensor..."

I have omitted info on the basic injection duration, but they are set by input to the ECU from engine RPM, Air Flow Meter, & Manifold Pressure sensors, which may not have been tampered with, but these are also worth keeping in mind while troubleshooting.

Let us know how you resolved this so we will all be the better off!

Good Luck, and hope this helps.

[adrian torres]

Update;

I still have the lowered mpg for city driving which seems to hover around 16mpg but I still get 25mpg for highway driving. The only update is that my radiator has clogs and needs to be replaced. I have not gotten around to it yet, but it is preventing hot water to get to the heater core. Now, I replaced the temp coolant sensor last year but it did not do anything. As I do not have access to the manual anymore, does the coolant that runs through the heater core come in contact with that sensor since it is top mounted? I am wondering that if coolant is not going through that part of the system if the coolant temp sensor is not getting any coolant to get a reading off, it may think my engine is cooler than it really is and thus run the car richer. When I drive for a long period of time I hear sloshing sounds then my heater works after 20 minutes of driving. Otherwise I get cold air. I have put it off as my car was not overheating. Lexus quoted me $800 for the radiator replacement with a coolant system flush, it that a good price?

I am thinking of getting this done, as the drop in mpg seemed to happen when my heater stopped working.

Any thoughts?

Adrian

[AHJGTA]

Do the coolant flush first - it might cost about a $100 at a small shop. Putting off cleaning the system may lead to bigger / more expensive issues as my assumption is that it's not good for the car to be not getting cooled in parts which are expecting it.

Make sure to use the approved coolant (usually red) and distilled water when refilling. If doing it yourself, then the flushing liquid should also be compatible.

A clogged radiator would not warant replacement. Usually it has to be damaged with rust or broken parts. The clog may have occurred due to using tap water instead of distilled.

[landar]

If you hear 'sloshing' and the heater is not always working, that strongly suggests air in the system. And yes, having an air bubble trapped in the system can fool the temp sensor. So, I would make sure you get the proper coolant level in there and make sure the system is not leaking, no air in the system. A clogged radiator will usually result in overheating and your symptoms do not indicate that.

[Exhaustgases]

$800 for a radiator is a bit high. But it would be an oem part. I bet you could still get oem if that is what you want online for better price. To change radiator is pretty easy. I don't remember for sure but Oriellys one I did a few years ago was about 130 or so I think.

[Exhaustgases]

A scanner would also show if coolant sensor is correct. And what the O2 sensors are doing, and like what others say LT and ST fuel trim.

[adrian torres]

Big Update!

I was able to change out my radiator at my dealership. After spending $800, I thought the problem was solved. But soon after, no only there was no change in the mpg, but the heater still was not working and there was still sounds coming from the behind the dash. After my car was returned twice to the dealership, bleeding the system was not working. But, they stated that my heater core was clogged and would be $1400 to replace the core and the heater valve to replace the problem. After a talk with the dealership, they were able to pressure wash the heater core system and flushed out a chunk of sealant that was used on my last 90k service that had caused the heater to stop working. There is still some sediment in the system, but, the heater was working again. I still heard some noises after the job but the lead tech assured me that the sounds will go away as the last of the sediment and blockage will work its way out. So far my mileage seems to have improved, but I will post once I am sure of it.

Also, I heard that my faulty cd player can be fixed by securing the rear connections through the glove box, instead of removing the unit. I will try and post this result on my other posting.

[AHJGTA]

Good timing and congratulations on getting the heater going.

This is the season to make us appreciate it.

[adrian torres]

Here is an update regarding the mileage and coolant system issues.

After the repair it has been two months of daily use and the mpg has improved slightly as an average. The best mpg happens (Other than highway) when the heater works rather than blows cold air. I was explained that my water flowing sounds behind the dash are not air pockets but coolant that is returning to the coolant circuit. The return flow of coolant does flow into the engine temp sensor. (which I replaced), thus causing a drop in mpg. The lower temp of the coolant or lack thereof is making the engine think it is cooler than it really is.

So it seems, that the only long term fix is to replace my heater core, which will not only fix my city mpg but the lack of hot air. The clog usually takes several minutes before allowing coolant to pass, depending if my trips either involves starts and stops or has periods of constant running at 2000 rpm. Once the noises appear, my city mileage returns to 19-20 otherwise it is 14-15mpg. At a $1400 repair job and gas being cheaper, the economics will have to wait until next winter (I live in Texas) or gas goes back to $4/gallon.

[landar]

I remain skeptical of the heater core causing your low MPG. For one, there is a heater valve which is in series with the heater core which controls the flow of coolant to the heater core. So, in the summer, when you are not using heat, the valve would be closed such that very little coolant flows thru the core. Would that sound like a smart design decision to make the coolant temp sensor dependent upon the position of the valve and the resulting coolant flow thru the core? No. A closed heater valve is similar to a "clogged" core. So I dismiss the notion of a clogged heater core causing the MPG drop.

One thing that has not been mentioned, unless I missed it (entirely possible), is if the thermostat has been changed. A stuck open stat could cause a cool engine and your low MPG.