Odd misfire intermittent problem climbing hills

[ledsales]

Have looked through a lot of threads here on misfires and nothing really matches my problem that I could find. I have a 94 Liberty GX auto wagon that has somewhat reduced power over the last few weeks/months (might just be aging as it is up to around 180k km). To go along with this, it has been suffering a misfire when under heavy load for more than about 30 seconds, although it varies.

 

As an example, going up a steep hill it will be ok until a misfire suddenly starts up and power dramatically drops off. This usually happens near the top of the hill, so it struggles to the top and once there, the misfire continues for a few seconds or so and then vanishes.

 

It doesn't happen every time, and sometimes it will happen on a particular hill once and then not the next time. I was thinking there might be a fuel pressure issue so replaced the filter, which was pretty old anyway. The other reason I am thinking this is that I am sure I used to be able to hear the fuel pump ticking in the tank when the ignition was first turned on. I can't hear that now.

 

So, just asking this in case anyone has come across this before, might save me lots of stuffing around and expense, as I don't have time to pull the car apart sensor by sensor looking for it, and as it is our only vehicle, it needs to get fixed asap. Am I right in thinking this could be fuel pressure related? Or, possibly a clogged cat?

 

Sorry, I have no codes etc to offer as the car has been pretty flawless for the last decade of use and hasn't needed much in the way of repairs, so I have no test gear for it. The CEL does not come on when this happens. The CEL has flashed on occasion but only for a second or two, and often with months between appearances, so it hasn't been worthwhile investigating so far.

 

Any help would be appreciated, otherwise I will have to let the local garage mess with it.

[uniberp]

New plugs and wires? I was surprised to learn that plugs are only good for about 50k on my 2008.

 

A marginal condition can be exacerbated by changes in load/heat/atmosphere/humidity.

[1 Lucky Texan]

does a 94 in this market have a knock sensor? could be cracked/bad.

[Bushwick]

ALWAYS do a full tune-up before assuming the worse. That means plugs, wires, air filter, FRESH oil change and fuel filter (which you've done already obviously). Reset battery cable and take it for a 50 mile drive. If it was running too rich from weak spark, it'll need the 50 miles roughly to clean out any soot build up on the O2 sensors, etc. Don't be afraid to drop it in "3" and drive it at 65 on the highway. "3" will pull it out of overdrive and kick the rpms up a bit.

 

Plugs can last 20k miles or stretch to 50k miles, just depends. I've had a misfire in another car that ended up being plugs with less than 30k miles on them. They were a little rounded on the tip and were correct heat range color. But were worn enough to cause a misfire.

 

You can do a full tune-up for roughly $50, and it'll make the engine run better, get correct MPG, feel more responsive, and stay healthier, longer. Do this, drive 50 miles, then see if it's still having a random misfire.

[ledsales]

Thanks for the replies guys, will definitely do plugs and HT cables asap, oil/filter was done a month or three ago and seem to recall the air filter was done not that long ago, but will check it. Still wondering about the fuel pump, as mentioned, the misfire isn't random, it only happens when under a heavy load for 20-30 seconds, then when you back off, the misfire vanishes. It's as if it is running out of fuel and going into a lean misfire, backing off greatly reduces the fuel rate required and so the misfire disappears. That's just what it feels like.

 

Re my thoughts on the fuel pump, does it normally make an audible tick when ignition is turned on? It definitely doesn't do that now, and so was wondering about the pump. When I changed the filter there was no pressure in the lines at all, although it had been sitting for an hour or so.

 

Re the knock sensor, would that be locate at the rear of the block at the top? There's a sensor in the block there which is not the temp sensor by the look of it, and it has a plastic case that is definitely cracked.

Edited April 19, 2016 by ledsales

[ledsales]

Ok, just did the air filter (a bit grubby, but not bad) but the interesting thing was when I did the plugs. The first 3 were fine but the 4th one (rear RH side) was rather interesting, as can be seen in the pic. At some stage that plug hole was stripped and some bozo tapped it out, ground down a larger plug and tightened it up using a screwdriver/cold chisel and hammer by the look of it, although those two indents on the plug look almost machined, so maybe a special type of plug for something else?

 

Anyway, looks like a more expensive job than I wanted, will see what the local mechanics have to say about it. Interestingly, it means I haven't changed the plugs since I bought the car 12 years ago, was sure I had done it but memory must be failing. So, all things considered, she is running pretty well!

 

But, just confirms that I really need an EV now (sorry if I'm blaspheming, I know enthusiasts like their dino burners!)

 

Edited April 22, 2016 by ledsales

[Fairtax4me]

Good bet the spark plug is your problem. I would be inclined to pull that cylinder head off and just put a used head on it. Of course I would be doing the work myself, which makes it much more cost effective. 

To pay a shop to put a used head on may not be worth it to you depending on the labor rate. 

 

Nothing wrong with electric vehicles! If I could afford one I would probably buy a Tesla. EVs will get more popular in years to come as younger generations begin driving them. So will autonomous vehicles, because apparently people who drive these days want to pay even less attention to whats going on around them and more attention to their cell phones and computers/tablets. Wont climb on that soapbox right now though. 

[CNY_Dave]

One way to try getting that plug out- piece of pipe the right size with some serrations in the end to 'match' the gouges, wedge the piper against the fenderwell, and turn...

 

Unless- is this one of those where if you pull the valve cover the plugs become exposed? Pipe wrench might do it then!

[AdventureSubaru]

Plugs and wires. I'd give it a 95% chance it will go away with those replaced.

[Bushwick]

I'm betting someone had an incorrect socket that was to large to fit in the recess of the head and they tried using a smaller 12 point socket that rounded the outer hex edges and they resorted to a hammer and chisel to tighten it, or possibly they couldn't get it out and left it in.

 

If you have a correct socket, it should come out even if you have to tap the socket on to clear the damage. Might want to get engine hot and up to temp, shut it off and pull wire, then try cracking it loose. Leave plug in the head until engine cools, then thread out rest of the way.

 

Also, be SURE to run new plug wires. They do go bad over time.

Edited April 22, 2016 by Bushwick

[ledsales]

The round plug has no flat sides at all, it's as if it was machined round, I ran a metal prob around it and it feels smooth. You can't tell from that pic, but the insulator is actually shorter on that plug than the others, and the metal body is deeper, so definitely a different plug. My correct plug socket just spins around on the dodgy plug, there's no sign of it grabbing on any part of the plug, which confirms what I can feel with the probe.

 

Unfortunately we only have one car and are rural, so I can't afford to do it myself as if anything goes wrong and I need to get parts, I have to wait for them to be couriered, which could mean days, which we simply can't be without a car for. I did think about using a purpose made tool to remove it, but if I break the plug and still can't remove it, I'm kind of stuffed. I will get the mechanic to quote on a quick head swap, but not sure how common heads in good condition for these are, not many wreckers yards around here.

 

Re performance, when I swapped the first 3 plugs, we then did a 120km round trip. On the way out it struggled to get up one of the larger hills, but on the way back there was definitely more power, and today it was feeling a lot more like it used to, so seems things are coming good with the new plugs, at least on 3 cylinders, so I think this last cylinder really needs to be done, unless I can win lotto and buy a new suby!

Edited April 24, 2016 by ledsales

[naru]

I doubt very much the plug is your problem.

Bad plugs do not wait 30 seconds under high load to misfire.

Rather,misfires at the top of a hill are CLASSIC symptoms of fuel starvation.

I would monitor fuel pressure as the misfiring occurs.

 

I would try to change the plug regardless.

[1 Lucky Texan]

maybe some size 12-pt socket of appropriate size - that is, slightly too small to drop over the plug, could have the 12 'points'/'lobes' filed/ground to create an 'entry' or ramp - on each lobe.

 

Them, hammer it onto the plug. Might grip well enough to extract the plug. This is similar to how thieves defeat security lug nuts.