More on this bucking issue

[jseabolt]

I've posted this before and thought I might recap on it.

 

The car: 2003 Baja, 2.5 non turbo, 5 speed, 77,000 miles (Yeah really)

 

The problem: Car sometimes bucks between 2500 and 3000 RPMs with foot slightly rested on accelerator. If I shift up to the next gear and lower the RPMs where I have to put more pressure on the accelerator to maintain the same speed, the bucking goes away.

 

Otherwise the engine idles fine, accelerates smoothly and has never stalled. It's a "cruise" issue.

 

The weird thing is if I set my cruise control so the engine runs at 2700 rpms  (just to see if there is a difference). the bucking goes away.

 

I'm somehow lead to believe it has something to do with the throttle mechanism.

 

I popped the hood and noticed both the mechanism that throttle cable is attached to and the mechanism the cruise control cable is attached to moves side to side. I know there is supposed to be some slack in the cables so that's normal but should there be side to side movement? The cruise control cable seems worse.

 

Also I checked to see if the nut had backed off but when I turn it with a wrench it turns the throttle shaft with it so the nut is either bottomed out or must have LockTite on it. I have never monkeyed with it.

 

Please watch this video and see what you think:

 

https://www.youtube.com/watch?v=P2zJ0eDs318

 

Couple of thoughts:

 

Could there be a dead spot in the throttle positioning sensor?

Could the throttle body be coked up with oil residue from the crankcase ventilation system?

 

Once a year the air auxiliary valve hiccups and kicks the check engine light on. I just reset it with my code reader and it stays off for another year. I understand this valve controls the idle speed? I'm not having any idling problems. I'd imagine it's not uncommon for a 13 year old car to hiccup occasionally.

 

 

 

 

[subnz]

Haven't had this happen but  suggest / I would use aerosol upper cylinder cleaner (in throttle body)   seafoam?

  intake manifold (take off pcv valve -clean, clean out hoses if slugded,  spray cleaner thru pcv valve hole into inlet manifold with engine running  (have to turn up idling speed to do this) - ie a little piece of wood btwn/in throttle stop linkage.

Also suggest throw in a bottle of injector cleaner in to fuel tank for good measure as well.

 

If this doesn't help agree it could be faulty throttle position sensor

 

Play (side play) in linkages looks normal. my previous 2.5 manual like this also.

Edited July 23, 2016 by subnz

[jseabolt]

I didn't want to mention this because it seems totally unrelated but this bucking is much worse after: 

 

1) I top the tank off with gas. Bucks for about 5 miles then either goes away or if I fill up on the way home from work, park the car overnight and then it's fine the next day.

2) If the car has sat more than 2 weeks.

3) Sometimes bucks at half a tank, sometimes worse with the low fuel light on.

 

I want to blame ethanol. After all it absorbs water and maybe if some of it has settled to the bottom of the tank explains why it bucks until the fuel in the tank is stired up after it runs for about 15 minutes.

 

I've tried running the tank down to near empty and running 100% gasoline to no avail.

 

Someone gave me instructions on how to check the fuel evaporation system but have not gotten around to it yet.

 

OH, one day when the car was bucking, I tried driving around with the gas cap loose to see if this had any effect but it didn't seem to. The check engine light never kicked on!

[subnz]

Another thought - do  you keep your tank full / topped mainly  

 

if its half full /less most of the time and car lives outside there could be condensation issues / water in fuel along with the ethanol thing.

 

 so throw in a quart of methyl alcohol - for 1/2 tank fuel to help disperse water / consumed through motor.  

 

 This is what I do especially after winter  ( or some bottle of trick juice that does the same thing but meths is cheaper)

 

Or fuel filter partially blocked (in tank I think - never changed one)

Edited July 23, 2016 by subnz

[jseabolt]

I topped the tank off back in May and maybe put 20 miles on it since then. I got it out in June to pick up some mulch. Then got it out last week. My Subaru sits in my garage at all times when not being driven.

 

I know it may seem like I don't drive this car enough but I have a bunch of "classics" I drive in the summer. When winter comes my Subaru is my daily driver. My "family" vehicle is a 98 Chevy conversion van. I prefer my 4 year old daughter not ride in my Subaru because she makes such a mess.

 

My Fiat 124 Spider:

 

 

My 1981 Trabant 601:

 

 

 

 

I had an issue with my riding mower this spring. I normally top the tank off before putting it up in my yard barn but forgot to. Actually there is more to this story than simply forgetting to top off the tank. More like my wife pissed me off because she said the leaves did not need mulching in December so I drove it straight to my yard barn and parked it.

 

Anyway I made a few laps and my mower started to sputter. I thought it was "stale gas" but when I disconnected the line, fuel just dripped out of it like the filter was clogged. So I replaced it with this one:

 

 

I continued to mow for 3 hours (I have a big yard) with no issues. I parked it in my yard barn and got it out 2 weeks later. Same thing happened!

 

This was a new filter, I see nothing in it but notice how fuel is puddling inside it.

 

 

I came to the conclusion that water was blinding over the filter element not allowing gas to pass through it.

 

I siphoned all my gas out of my tank and what was left in my gas can and ran it through one of those Mr. Funnels:

 

 

It's supposed to filter water out of the fuel. I actually tested it one day using two glass jars and sure enough it does work!

 

I still have that filter. It's dried out now and I can blow through it.

Edited July 23, 2016 by jseabolt

[Fairtax4me]

Ethanol is really kind of horrible for the fuel system. It corrodes parts and causes all sorts of running problems. It has much lower heat capacity than straight gasoline, so we get less power production from ethanol gas than from straight gas. That means higher fuel consumption due to lower efficiency. It's especially bad if you leave it in the tank for long periods. If you run through a tank in a week or two and fill up again with fresh gas it's not going to cause too much of an issue. But the same gas in the tank for a month or more will start to cause problems.

Alcohol absorbs water. It acts like a conduit if you will, between gasoline and water and allows the two to mix. Products like Heet, Dri-gas, etc., are all alcohol based because alcohol mixes with and absorbs water. So if there is water in the fuel it gets mixed in with the fuel which allows it to burn.

Ethanol works in much the same manner, absorbing water and allowing it to burn with the fuel. But ethanol tends to be about 10% of the fuel we get these days. Which means the fuel can absorb and hold a LOT of water.

 

Here's an experiment you can do to see how much water is in your ethanol gas. Take a large test tube or graduated cylinder and fill it with 100ml of ethanol gas. Add 10ml of water, cap the cylinder and shake it for about 30 seconds. You'll have to carefully vent some pressure from the cylinder every few seconds because gasoline is volatile and shaking it will cause it to evaporate and pressurize the cylinder.

After 30 seconds, put the cylinder down and let it settle for a few minutes.

 

Water will settle to the bottom of the cylinder and how much depends on the water content already in the fuel. The extra 10ml of water will saturate the ethanol and any water that it had absorbed will separate and drop out.

Let say you now have 17ml of water in the bottom of the cylinder, that means 7% of that fuel was water.

 

Now how does that relate to the fuel in your cars gas tank? When a car sits the repeated heating and cooling of the atmosphere can cause moisture to condense inside the fuel tank. That moisture will tend to get absorbed by the ethanol in the gas, but only up to a certain amount depending on the ethanol content. Once it exceeds that, you start getting the same kind of drop-out as what happens in the test tube cylinder.

I've replaced quite a few fuel pumps that had clear water lines on them where the water in the bottom of the tank had caused the pump to corrode and eventually caused it to fail. That's generally in vehicles that have been sitting for several months, or in the worst cases over a year.

If your car sits for long periods without use, add an ethanol stabilizer treatment to the fuel before filling up to help prevent the problems that ethanol causes.

 

As to the cause of your bucking issue. Kinda sounds like a misfire. Most of the time a part the throttle misfire will be caused by worn spark plugs. How old are the plugs and wires?

Edited July 23, 2016 by Fairtax4me

[jseabolt]

Sorry for the late reply. My spark plugs and plug wires are not *new* but have maybe less than 10K to 15K miles on them. And a new fuel filter.

 

One other component comes to mind. I'm wondering if the TPS has a dead spot in it? The bucking seems to occur when the accelerator is in the same position. About the same position with my foot resting on the accelerator.