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RobMD 10-05-2011 05:18 PM

'99 Rough idle/Misfire 2.5L
Hey guys,

This would be my first post but I'm no stranger to a Ford Ranger. I've owned mine for 7 years now since it had 38,000 miles on it. It's a 5 speed 2.5L multi-port extended cab 2WD. It now has 139,000 miles on it and is a daily driver.

My problem has existed for years. I've pondered, thought and thought some more about my problem. Yes, being a mechanic at Deere, I'm quite capable of maintaining my truck, 3,000 mile oil changes, timing belt changes, new water pump... I could go on and on about all the maintenance i've done to it.

A quick search in the forum archives about my specific problem showed a lot of compression tests, fuel filters, IAC tests and EGR replacements that solved nothing for the owner. I suspect it's one of those crazy computerized dilemmas which have no end.

So.... maybe someone can put some new ideas into my brain.


Engine starts and runs normally when cold. After 5 minutes of driving and approaching a stop sign, engine begins loping and shaking the whole truck. People in my rearview mirror have actually pointed and mouthed "it's a shaking truck." This shaking affects how I leave off in 1st gear, and often causes concern when trying to pull out onto a busy street.

Note that the engine will smooth itself out and idle normally as if my problem doesn't exist. Slap the gas pedal and voila, the mysterious shaking returns. At 1/4 to wide open highway speed throttle ranges, the engine runs smooth.

The problem is magnified when the engine is cold. When it's warm, the problem is less and less apparent. When the weather outside is cold, the engine takes longer to stop misfiring and running rough but eventually goes away.


Only one code has been coming up the last 30,000 miles: Cylinder 2 misfire.


IAC and throttle body cleaned half a dozen times over 100,000 miles.
EGR vacuum lines checked and EGR cleaned/checked.
Spark plugs and plug wires changed twice.
MAF sensor unplugged/cleaned - MINOR difference.
Fuel filter change 20,000 miles ago.
Timing belt at 120,000 miles and has made no difference in running - static timing is fine.
Antifreeze changed and waterpump replaced.
Battery and alternator were replaced once (unrelated)
Sprayed WD40 around all vacuum areas - no leaks.
No leaking exhaust, cat not plugged.
Air filter is standard motorcraft paper. I got rid of the K&N because I believe it could have damaged the MAF sensor with all that filter recharger crap.


K&N air filter might have damaged MAF sensor? There is no code relating to the MAF sensor.
Problem is INTERMITTENT and would NOT be a mechanical problem. If the truck did the same thing 100,000 miles ago, and is no different today, why would it be mechanical?
Problem has a little to do with engine TEMPERATURE.


...I HOPE!

Thanks in advance for your input.

bamuth 10-06-2011 06:57 PM

Re: '99 Rough idle/Misfire 2.5L
I see you haven't change the coil packs yet, have you had them diagnosed yet or checked to see if number two cylinders coils are faulty in any way. my truck does just the opposite, when I push in the clutch at times when it's really humid or hot out it idles higher then crap and then slowly goes down.

sheltonfilms 10-06-2011 08:49 PM

Re: '99 Rough idle/Misfire 2.5L
A free test you can do since its a dual spark design is to swap the coil packs. One is the primary and the other is just a "waste spark" pack.

So when is the problem occurring? You said its fine when cold and then in another sentence you said its magnified when cold.

Depending on which one it is, you are either having problems in open loop (when engine is cold) or closed loop (engine is normal temp).

SalRanger2 10-06-2011 08:51 PM

Re: '99 Rough idle/Misfire 2.5L
head gasket maybe?? could cause the missfire code too i believe

RobMD 10-07-2011 12:40 PM

Re: '99 Rough idle/Misfire 2.5L
Hey guys,

Thanks for the replies.

I haven't even thought about the coil packs - Shelton, I never even knew that the multiport 2.5 works that way... could you explain more? I THOUGHT that BOTH plugs on each cylinder fire at the same time.. so you're saying they do not?

As for the cold running, I thought I had made myself clear - but anyway, the engine starts misfiring/shaking at idle COLD. The colder it is outside (and the colder the engine is), the harder it shakes and the longer it takes for it to go away.

Once the engine has warmed up (radiator reaches temperature as well) the unit will hardly EVER shake/misfire at idle.

IF IT WERE COIL PACKS... why only at cold and at idle?

sheltonfilms 10-07-2011 02:16 PM

Re: '99 Rough idle/Misfire 2.5L
For the dual spark design:

The exhaust side plugs are the plugs that fire during the compression strokes. The intake side fires only on the exhaust strokes to burn the unburned gas, for emissions.

When cylinder 1 (exhaust side) fires for the compression stroke, cylinder 4 (intake side) is firing on the exhaust stroke. 1 and 4 are paired and so are 2 and 3.

You can swap plug wires for the intake side to have them fire at the same time. I've done this and have noticed quicker starts and smoother idles, and just a hair better performance.

Here is a link about it:

Heat changes electrical characteristcs such as resistance and inductance. Very important for high energy spark.

Like I said you can try swapping the coil packs and see if this changes anything.

RobMD 10-07-2011 06:28 PM

Re: '99 Rough idle/Misfire 2.5L
I didn't realize how that worked. They really outdid themselves for the emissions on these 2.5L's, didn't they? Looks like the term "multiport" is useless to anyone with horsepower on their minds.

Well thanks! I'll definitely give the swap a try.

I wouldn't think a cylinder #2 misfire would randomly come up. I assume something is wrong with that particular cylinder's firing ability. The CEL comes and goes... but the code has always the same for the past two years.

sheltonfilms 10-07-2011 07:47 PM

Re: '99 Rough idle/Misfire 2.5L
It wasn't just on the 2.5s it was on the 2.3 Rangers from late 80s up and 2.3 mustangs early 90s and up.

BTW the 2.5 is the same block as the 2.3 just a different crank and rods (its stroked), in case you didn't know that already.

RobMD 10-08-2011 08:11 PM

Re: '99 Rough idle/Misfire 2.5L
Nope, I didn't know that. For some reason, it looks like in '02 they went back to the 2.3L. Looked under the hood one time, didn't look like it had multiport ignition.

As far as i'm concerned, multiport is B.S.

sheltonfilms 10-08-2011 08:13 PM

Re: '99 Rough idle/Misfire 2.5L

Originally Posted by RobMD (Post 579810)
Nope, I didn't know that. For some reason, it looks like in '02 they went back to the 2.3L. Looked under the hood one time, didn't look like it had multiport ignition.

As far as i'm concerned, multiport is B.S.

They changed in mid year 2001. before 2001 1/2 it was the SOHC Lima after 2001 1/2 they change to a DOHC Duratec engine (totally different).

Alot of cars now and days have dual spark setups. The new Challengers do (imagine paying for 16 platinum spark plugs when doing a tune up.) Dual spark was developed by the Japanese a long time ago.

Any luck with the swapping?

RobMD 10-11-2011 05:39 PM

Re: '99 Rough idle/Misfire 2.5L

I swapped my coil packs. No difference even with disconnecting battery before repair. If I take the #2 plug wire out of each coil, there is an arc about 1 inch long and when the engine is revved and allowed to return to idle, there is spark no matter if the truck is shaking or not.

I decided to put my snap-on timing light on the #1 cylinder (on the POWER STROKE coil side) and found that when the engine is running smooth at idle, it stays right about where it should be. Now when I let it start shaking, the timing ranges from 20-30 degrees out and is completely erratic. Now what on earth can be causing this spark to register out of time at idle, and then go back to being correct?

I decided to check the coolant temp sensor ranges at hot and cold - correct resistance (3.2K at 190 degrees).

Cycled TPS - range is correct

Checked MAF sensor - there is a resistance of 135,000 ohms through MAF SIGNAL and MAF SIG RNF (in other words, through the resistor).

I cleaned the MAF sensor yet again, and it made a tiny bit of difference. I swear to gosh that thing is overly sensitive. I blew a tiny tiny bit of air from my mouth onto the sensor from a 10 inch distance and the truck idled down slowly then abruptly quit.

So, I'm still leaning towards some kind of MAF problem or an injector problem.

I still am puzzled at how, when warmed up, the truck will stop shaking at idle. However, it will shake if you slap the gas pedal when warm, but will quickly recover.

This is a serious issue primarily because it affects how I leave a stop sign and pull out onto a busy street. The Gear shifter often bounces side to side about 4 inches - that's absurd in my opinion. :banghead:

sheltonfilms 10-11-2011 06:39 PM

Re: '99 Rough idle/Misfire 2.5L
It would probably die out if you blew on the sensor.


That sensor relates the temperature drop of an element to determine how much air is flowing. It takes this measurement through the tiny hole. Then it pretty much scales the value to associate with the whole size of the MAF.

So when you are blowing on that tiny sensor area, the truck is really thinking its getting a whole lot more air than it really is. So you blowing just a tiny bit would make the computer think that you are essentially blowing what a leaf blower would be blowing into the engine. So it floods the engine (rich). Plus you have to take into account that the throttle body is closed at this time, so even if you was blowing that volume it would never reach the cylinders

Did you gapped the spark plugs? Some people say they are pre-gapped but they really aren't. You should always check the gap before install. Just checking

RobMD 10-18-2011 04:09 PM

Re: '99 Rough idle/Misfire 2.5L
Thanks for the lesson! Makes sense to me. I knew it had something to do with how quickly the filaments are cooled in the MAF sensor but didn't really grasp it.

Yeah, I gapped the plugs correctly per the little decal / emissions sticker on my front clip. I changed my plugs and wires at 60,000 miles, and then at 120,000 miles. I first used napa wires (motorcraft plugs) then went to motorcraft wires (autolite plugs).

The plug and wire changes/differences made absolutely no difference in how the truck performs at idle. I cleaned the throttle body and IAC again and, no difference. I could put in the world's greatest plugs and wires and there won't be any difference.

I guess if I've been driving it like this for roughly 80,000 miles, why can't I drive it for another 80 grand like this? I guess things slowly work on my nerves...

Maybe one day i'll just buy a stupid new MAF sensor and slap it on there, if that helps.

sheltonfilms 10-18-2011 05:08 PM

Re: '99 Rough idle/Misfire 2.5L
One guy had a bad oring on his 2.5 and gave a bad idle, after replacing a lot of parts it was still misfiring. Did the comp test and found the problem.

I would try doing a compression test. May have leaky valve, sleave, or ring. Worth a shot.

ekandresen 02-02-2012 10:46 AM

I have the same problem and have replace multiple parts with no change. Now it has started to stall on cold soak restart. Seems like this is a common problem, but I have yet to find a solution.

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