Cause of low power, bad mpg, and barely passing smog?

[yotatake2]

I recently got my truck smogged and it barely passed by the skin of it's teeth. When I got it smogged a year ago, it passed with flying colors. I've also noticed that the mpg's have dropped significantly (I'm now in the low 14's) and it doesn't have nearly as much power as it used to. I just replaced the spark plugs and O2 sensors but these problems still persist.

My question is, what are other sources of these symptoms that I could start looking into?

Thanks

 

[toyman151]

how many miles on the motor?

 

[gamefreakgc]

Here's a few places to start, all of these components are related to air/fuel control:

1. MAF cleaning - search this if you need a how-to on what parts to clean. The wires will get clogged up over time.

2. Air filter - a paper filter should be replaced every year if it's driven consistently, less often. Sometimes leaves and other materials will clog up the filter prematurely and kill your MPG's.

3. Spark plug wires - usually if these go bad it'll throw a check engine light, but if they are old then replacing them can improve spark quality.

4. Spark plugs - you did use double-ground plugs right? NGK or DENSO ones are the only ones to be used in these engines, the rest are inferior.

5. Fuel filter (not likely) - A clogged fuel filter can also cause some funny issues but usually the truck will run lean or sputter under load.

6. Catalytic converter - A clogged cat or one on it's way out will ruin you mileage due to the rear O2 overcompensating with more/less fuel. The only good sign of a failing cat is the rotten eggs smell from the exhaust but usually your truck will throw the P0420 before it gets that far. Still, your exhaust should not have any odors when warm and have a slightly sweet smell to it compared to other vehicles.

7. Last but not least, valve clearances - As the engine ages the valve lash will change over time. It may get to the point that the valve do not seal completely, causing a loss in compression (less power) as well as letting unburnt fuel or oxygen downstream to the sensors. As you might guess, the ECU then overcompensates and makes the issue worse. Correcting the valve clearances by installing the correct size shim gets it back in order again. Worked wonders on my previous truck and eliminated my CEL lights that seemingly had not cause and increased the power back to where it should have been.

 

[Paperjam]

Here's a few places to start, all of these components are related to air/fuel control:

1. MAF cleaning - search this if you need a how-to on what parts to clean. The wires will get clogged up over time.

2. Air filter - a paper filter should be replaced every year if it's driven consistently, less often. Sometimes leaves and other materials will clog up the filter prematurely and kill your MPG's.

3. Spark plug wires - usually if these go bad it'll throw a check engine light, but if they are old then replacing them can improve spark quality.

4. Spark plugs - you did use double-ground plugs right? NGK or DENSO ones are the only ones to be used in these engines, the rest are inferior.

5. Fuel filter (not likely) - A clogged fuel filter can also cause some funny issues but usually the truck will run lean or sputter under load.

6. Catalytic converter - A clogged cat or one on it's way out will ruin you mileage due to the rear O2 overcompensating with more/less fuel. The only good sign of a failing cat is the rotten eggs smell from the exhaust but usually your truck will throw the P0420 before it gets that far. Still, your exhaust should not have any odors when warm and have a slightly sweet smell to it compared to other vehicles.

7. Last but not least, valve clearances - As the engine ages the valve lash will change over time. It may get to the point that the valve do not seal completely, causing a loss in compression (less power) as well as letting unburnt fuel or oxygen downstream to the sensors. As you might guess, the ECU then overcompensates and makes the issue worse. Correcting the valve clearances by installing the correct size shim gets it back in order again. Worked wonders on my previous truck and eliminated my CEL lights that seemingly had not cause and increased the power back to where it should have been.

You are correct in all except for the catalyst. The downstream O2 is simply there as an emission requirement for catalyst monitoring. The upstream O2 is the one that adjusts fuel trims

A plugged cat won't neccisairily throw a p0420, but it will restrict the engine and cause the symptoms he is describing. However an ineffective catalyst will because the downstream O2 sensor wil start to show the presence of fuel, thus the p0420

 

[Paperjam]

I recently got my truck smogged and it barely passed by the skin of it's teeth. When I got it smogged a year ago, it passed with flying colors. I've also noticed that the mpg's have dropped significantly (I'm now in the low 14's) and it doesn't have nearly as much power as it used to. I just replaced the spark plugs and O2 sensors but these problems still persist.

My question is, what are other sources of these symptoms that I could start looking into?

Thanks

Could you post your smog results please OP? I'm curious to see the gas analysis.

 

[yotatake2]

how many miles on the motor?

220K. Got it with 180K.

 

[yotatake2]

Here's a few places to start, all of these components are related to air/fuel control:

1. MAF cleaning - search this if you need a how-to on what parts to clean. The wires will get clogged up over time.

2. Air filter - a paper filter should be replaced every year if it's driven consistently, less often. Sometimes leaves and other materials will clog up the filter prematurely and kill your MPG's.

3. Spark plug wires - usually if these go bad it'll throw a check engine light, but if they are old then replacing them can improve spark quality.

4. Spark plugs - you did use double-ground plugs right? NGK or DENSO ones are the only ones to be used in these engines, the rest are inferior.

5. Fuel filter (not likely) - A clogged fuel filter can also cause some funny issues but usually the truck will run lean or sputter under load.

6. Catalytic converter - A clogged cat or one on it's way out will ruin you mileage due to the rear O2 overcompensating with more/less fuel. The only good sign of a failing cat is the rotten eggs smell from the exhaust but usually your truck will throw the P0420 before it gets that far. Still, your exhaust should not have any odors when warm and have a slightly sweet smell to it compared to other vehicles.

7. Last but not least, valve clearances - As the engine ages the valve lash will change over time. It may get to the point that the valve do not seal completely, causing a loss in compression (less power) as well as letting unburnt fuel or oxygen downstream to the sensors. As you might guess, the ECU then overcompensates and makes the issue worse. Correcting the valve clearances by installing the correct size shim gets it back in order again. Worked wonders on my previous truck and eliminated my CEL lights that seemingly had not cause and increased the power back to where it should have been.

I forgot to mention that i have also recently cleaned the MAF with MAF cleaner, and i changed out the air filter and spark plug wires when I did the plugs a couple hundred miles ago. Fuel filter is also relatively new. How involved is it to put shims in? I've worked with valves before but never on my daily so I'm a bit nervous about that. I need to do my valve cover gasket pretty soon too, so might as well kill 2 birds with one stone

 

[Hacker]

I recently had my '99 with 220K fail smog in Cali, and between cleaning the MAF and replacing the upstream O2 sensor, it passed with flying colors on the retest.

 

[ridefast]

Pcv valve?

 

[AZBub]

Any OBDII codes? Better to start there I suppose.

I agree with 1-6 posted by gamefreakgc.

I had horrible gas mileage and poor power. My issues were...

1.) 2 misfires - one bad spark plug the other was loose.

2.) bad Cat

got those fixed and boom. 19-21 mpg. I don't care if people don't believe it cause neither do we but thats what we get. Of course we drive it like a Snowbird Caddy.

 

[slapdappy]

7. Last but not least, valve clearances - As the engine ages the valve lash will change over time. It may get to the point that the valve do not seal completely, causing a loss in compression (less power) as well as letting unburnt fuel or oxygen downstream to the sensors. As you might guess, the ECU then overcompensates and makes the issue worse. Correcting the valve clearances by installing the correct size shim gets it back in order again. Worked wonders on my previous truck and eliminated my CEL lights that seemingly had not cause and increased the power back to where it should have been.

Please tell me more about this.
How does one check valve clearance? Will compression test alone tell me all I need to know?

 

[jhfleet]

Please tell me more about this.
How does one check valve clearance? Will compression test alone tell me all I need to know?

A compression test would show low compression on a cylinder that has a valve that's not fully closing. But, a good test is no assurance that valve clearances are all correct. However, I have some doubt that valve lifter clearance changes due to wear would ever result in a valve being unable to close due to inadequate clearance. My experience is the opposite in that high-mileage engines generally have too much clearance rather than too little and excessive clearance has no effect on valves closing. Excessive clearance causes lifter noise but has little or no effect on performance.

 

[gamefreakgc]

Pcv valve?

I forgot this! Yes this can also make a big difference.

Please tell me more about this.
How does one check valve clearance? Will compression test alone tell me all I need to know?

It will tell you some. To check valve clearances, you need to remove the valve cover and measure the distance between the camshaft (with lobe pointed away from the engine) and the shim. Normally this is done when you change the valve cover gaskets since it's a fair amount of work, plus the old gaskets might leak if they are old and you take them off. It's best described in the FSM or repair manual if you have one, it's kind of complicated.

A compression test would show low compression on a cylinder that has a valve that's not fully closing. But, a good test is no assurance that valve clearances are all correct. However, I have some doubt that valve lifter clearance changes due to wear would ever result in a valve being unable to close due to inadequate clearance. My experience is the opposite in that high-mileage engines generally have too much clearance rather than too little and excessive clearance has no effect on valves closing. Excessive clearance causes lifter noise but has little or no effect on performance.

My last truck had valve clearances so tight that the valves were not seating and throwing a rich/lean code at the same time since the cylinder wasn't sealed. Basically had almost no compression. But that was at 270k miles with never being adjusted. That's rare, but it can happen. Exhaust valves tend to tighten and intake valves tend to loosen but every engine is different. The 5VZ-FE seems to be a pretty decent engine though and doesn't need to be adjusted very often.

 

[runner823]

You are correct in all except for the catalyst. The downstream O2 is simply there as an emission requirement for catalyst monitoring. The upstream O2 is the one that adjusts fuel trims
N
A plugged cat won't neccisairily throw a p0420, but it will restrict the engine and cause the symptoms he is describing. However an ineffective catalyst will because the downstream O2 sensor wil start to show the presence of fuel, thus the p0420

Not to be rude but you made everyone that read your reply to the OP that much dumber:Cry:
So I'll correct you. The downstream o2 can affect fuel trim by 5% +/- and in some makes and models up to 10%+\-. This can be viewed by looking at your total fuel trim on how much is being adjusted by the post o2.
The the post o2 monitors the amount of oxygen that cat is able to use in order to convert HC and CO to CO2. So when the cat start to deteriorate its ability to stores oxygen deteriorate. That's what the post o2 is monitoring during the p04## check. So where does the cat gets it oxygen? Well that's why the pre-o2 switches from rich to lean. The 14.7 is more for emission. Plus if the post o2 see raw fuel or high unburned fuel(misfires) the post o2 will attempt to protect the cat by adjusting total fuel trim. So the ECU may more likely set a p0171/p0174 and/or AF sensor DTC.
You should google p0420/p0430 run criteria. So you can further understand why a " present of fuel" WILL NOT set a p0420/p0430.

FYI: my background
- 20 year master ASE and ASE L1 advance engine performance
- Washington state master emission specialist from 2005 - 2010
- Toyota Master/MDT
- Lexus master
- Honda gold level technician

 

[runner823]

Could you post your smog results please OP? I'm curious to see the gas analysis.

:lol: why? Will you even know how to read and understand the results

 

[runner823]

I recently got my truck smogged and it barely passed by the skin of it's teeth. When I got it smogged a year ago, it passed with flying colors. I've also noticed that the mpg's have dropped significantly (I'm now in the low 14's) and it doesn't have nearly as much power as it used to. I just replaced the spark plugs and O2 sensors but these problems still persist.

My question is, what are other sources of these symptoms that I could start looking into?

Thanks

A good place to start is review the fuel trim data like short, long, and total. I would check it at idle and off idle under load to get a good understanding to what the ECU is doing. If you don't have access to an advance scan tool. I suggest you take it to a good repair shop like a Toyota dealership or an independent repair shop that specialize in Toyota. Please no Midas, Firestone, or Goodyear.

Good luck

 

[ridefast]

combustion gasses that enter the crank case (unberned fuel) are revealed by the PCV valve to keep pressure correct in the valve cover. The gasses are rerouted back to to intake and reburied. Someone might have a better explanation than I, but this is how I understand it from my dad.

Its really easy to change, location is on the front of the valve cover on the passenger side. My PCV was stuck closed and the grommet was dried out and cracked, I would think that the grommet would get ruined while taking out the PCV in most cases. I ordered the parts on amazon you will need the grommet too.
http://www.amazon.com/gp/product/B00GJJNQB6/ref=oh_aui_detailpage_o00_s01?ie=UTF8&psc=1
http://www.amazon.com/Beck-Arnley-039-6428-Valve-Grommet/dp/B000CRVFYQ/ref=pd_bxgy_auto_text_y

At the time I did the pcv valve i took the throttle body off and cleaned it (super dirty inside), found a loose hose changed spark plugs (used OEM) inspected vacuum lines changed the fuel filter and changed all fluids did a through cleaning of the MAF sensor. The PO had tried to fix the intake rubber boot with sensor safe RTV and missed a hole and the engine was pulling in air that wasn't being read by the MAF so I installed an airaid mit the replace the parts that were bad.
This is all being done because I am chaising a po171 code and because I dont know when the PO did fluids? Next is a fuel pump if cel comes back in a week.

Conveniently my OEM spark wires came in the mail after all this was done. My truck runs way better now.

 

[Paperjam]

:lol: why? Will you even know how to read and understand the results

You know correcting me is one thing, but being a cock about it is another. Why say not to be rude when you fully intend on it?

You would think for someone with so much experience you would allow someone else to be curious about it.

Yeesh.

 

[yotatake2]

Lots of good ideas here. I'm going to try cleaning the MAF again and maybe check out my pcv if I have time. I also just put on some Camburg UCA's and replaced all my front suspension bushings and got the truck aligned and I've actually noticed a bit of an improvement in power. I guess the alignment was just so messed up because of the bad bushings that it was actually robbing some power since the wheels weren't rolling completely straight.

 

[jgue467]

... noticed a bit of an improvement in power. I guess the alignment was just so messed up because of the bad bushings that it was actually robbing some power since the wheels weren't rolling completely straight.

I'm sure the fuel trim data would have shown this huh Paperjam? I'm with you man, don't like show-offs and/or bullies. whack em' in the head! :cowboy: