OHV misfire with no check engine light!?!?!

[eddie95]

I have been battling a slight misfire for a while. It generally consisted of a jerking motion whenever you try to accelerate hard and the actual misfire starts at almost exactly 2000 rpm. This misfire only happens under acceleration, so if you are just cruising, it is really smooth. The misfire is also not very consistent, as it will jerk every second or two instead of a constant shaking.

There is no check engine light, and I had ignored the misfire for a long time since it use to be occasional and I hardly ever drive on the freeway. Well today I drove on the freeway and I noticed the misfire more than usual and my gas mileage has taken a dump lately as well (used to get 22 mpg highway, now only 17). I can also smell a slight gasoline odor when the engine is cold and idling (however there is no noticeable misfire at idle).

I replaced the plugs and wires about 10 thousand miles ago with Autolite wires and NGK plugs, so I know that they are OK. The fuel filter was also replaced at that same time. Does anybody have any idea where I should start looking here? I had been running injector cleaner through at almost every fill up, but I haven't been doing that lately (and come to think of it, when I stopped was when I started having the problem). Should I just run some more injector cleaner through and hope that it clears out whatever is going on, or does it sound like an electrical problem?

I appreciate your help!

 

[eddie95]

I will get some MAF sensor cleaner tomorrow and try that since the sensor looks original at 130,000 miles. I also double checked the firing order, and it was correct.

 

[96eb96]

Yeah, definitely clean the maf.
A 95 is unlikely to get any misfire codes since it isn't OBDII.
Not saying it is your problem but there are some reports of misfires with using non motorcraft/autolite spark plugs and Motorcraft wires. Autolite wires are probably ok.
Are the NGK plugs anything special? Or just stock platinum replacements?

 

[eddie95]

Yeah, definitely clean the maf.
A 95 is unlikely to get any misfire codes since it isn't OBDII.
Not saying it is your problem but there are some reports of misfires with using non motorcraft/autolite spark plugs and Motorcraft wires. Autolite wires are probably ok.
Are the NGK plugs anything special? Or just stock platinum replacements?

You're correct on the OBD1 part (thats good information that the obd1 is less sensitive, didn't know that before). I may just throw a new set of plugs at it and see if that makes a difference. Also planning on testing the throttle position sensor. The last thing that I'm wondering if it could be would the the camshaft position sensor. I have had a slight squeak that comes from the front of the engine (trust me, I have tried EVERYTHING to fix it), and that is probably what is making the noise. But the squeak goes away in a couple seconds now so I doubt that is the problem.

 

[Runnin'OnEmpty]

.....This misfire only happens under acceleration, so if you are just cruising, it is really smooth. The misfire is also not very consistent, as it will jerk every second or two instead of a constant shaking.
......I replaced the plugs and wires about 10 thousand miles ago with Autolite wires and NGK plugs, so I know that they are OK. ......

An intermittant misfire like that can be caused by a plug
gap being too wide. When the throttle is opened cylinder
pressures increase, and that makes it harder for the
spark to be generated. Problem might be a weak spark
on one or more cylinders.

If you have a multimeter, this stuff is easy to check.
First check the plug gaps to ensure none are wider
than .052" or whatever your specs are. Then test the
resistance in the plugs and wires. Plugs will have about
4-8K ohms resistance, and wires will have around
6-10K ohms per foot of wire. If any plug or wire tests
out of this range you've found the cause of the misfire.

The problem could also be a weak coil pack. You can
test the coil resistance too, but I don't have the values
at hand right now...

 

[Runnin'OnEmpty]

Coil secondary resistance is 13-15K ohms.

 

[96eb96]

You're correct on the OBD1 part (thats good information that the obd1 is less sensitive, didn't know that before). I may just throw a new set of plugs at it and see if that makes a difference. Also planning on testing the throttle position sensor. The last thing that I'm wondering if it could be would the the camshaft position sensor. I have had a slight squeak that comes from the front of the engine (trust me, I have tried EVERYTHING to fix it), and that is probably what is making the noise. But the squeak goes away in a couple seconds now so I doubt that is the problem.

I have a feeling this is a plug/wire issue. In my experience going motorcraft never let me down. Sometimes it is even less than other brands with discounts.

There is also a possibility of a weak coil. Another major thing is using silicone tune up grease on plugs AND coil ends. If you didn't do that this is a good idea.

Spray your wires with a fine mist of water at night. see if there is any major arcing.

The cam pos sensor is in the rear of the engine. Did you remove your drive belt to eliminate all accessories? Also if that unit fails to the point where it would cause a misfire a code is often set.

 

[eddie95]

I have a feeling this is a plug/wire issue. Spray your wires with a fine mist of water at night. see if there is any major arcing.

I did degrease the engine a while back and it misfired really bad until all the water had dried off over night (only drove it back in the garage and was fine in morning).

There is also a possibility of a weak coil. Another major thing is using silicone tune up grease on plugs AND coil ends. If you didn't do that this is a good idea.

I did not do this, however the wires came pre-greased so I figured it was sufficient. The coil is a possibility for sure, I just really don't want to have to buy one

The cam pos sensor is in the rear of the engine. Did you remove your drive belt to eliminate all accessories? Also if that unit fails to the point where it would cause a misfire a code is often set.

You got me there, I really thought it was at the front. I think that I'm still fighting some glazed pulleys that were a result of using prestone belt dressing when I heard it squeak twice on a -10 degree day (don't ask, I was young and dumb ). I am running a gatorback now and that made a huge difference.

Well, I also have discovered that my IAC valve is about shot. I had gotten an extra from a junk yard a while back and cleaned out the original and the junkyard one. Hopefully I can find the other back and swap them out.

Also, I did put a high flow spectre filter on there a while back. That leads me to believe that maybe some extra particulates made it past the filter and dirtied up my MAF sensor. I have the cleaner now and I will try to clean it out this weekend. The sensor on the 95's is kind of a pain to get at so I couldn't do it tonight.

 

[eddie95]

UPDATE:

I realized that the MAF cleaning was way easier than I thought and I just got in from doing it as well as cleaning and lubing the current IAC valve. That thing hasn't idled this smooth since I've owned it. Not sure if it fixed the other problem, but wow! It starts way easier now.

 

[eddie95]

I drove it today and I thing that so far it is problem solved! That MAF was really gunked up and now it feels like I've got an extra 20 horsepower (no joke). I appreciate all of your help on here, and I will refer back to it again if the problem comes back up again. Maybe it can help somebody else out too.

Thanks

 

[96eb96]

I drove it today and I thing that so far it is problem solved! That MAF was really gunked up and now it feels like I've got an extra 20 horsepower (no joke). I appreciate all of your help on here, and I will refer back to it again if the problem comes back up again. Maybe it can help somebody else out too.

Thanks

A dirty maf is like getting a direct deposit statement from your employer that is 25% higher than the actual money sent to your account. Then you go and spend the money. That is pretty much what is going on! Sometimes you may get lucky and have some extra money there(the o2 sensors can compensate a bit) but usually it is not a good idea.

Since the air is being incorrectly reported, it is almost like having an invisible vac leak and the computer adds more fuel and all kinds of issues happen. A scan tool usually gives away a bad MAF by the baro parameter or out of whack fuel trims without vac leaks.

Problem is that sometimes cleaning don't help, and the MAF has truly failed. Without a scan tool people end up in an infinite loop of replacing parts.

This symptom tells it all:

It generally consisted of a jerking motion whenever you try to accelerate hard

If you feel anything like that - clean the MAF!

 

[eddie95]

A dirty maf is like getting a direct deposit statement from your employer that is 25% higher than the actual money sent to your account. Then you go and spend the money. That is pretty much what is going on! Sometimes you may get lucky and have some extra money there(the o2 sensors can compensate a bit) but usually it is not a good idea.

Since the air is being incorrectly reported, it is almost like having an invisible vac leak and the computer adds more fuel and all kinds of issues happen. A scan tool usually gives away a bad MAF by the baro parameter or out of whack fuel trims without vac leaks.

Problem is that sometimes cleaning don't help, and the MAF has truly failed. Without a scan tool people end up in an infinite loop of replacing parts.

This symptom tells it all:


If you feel anything like that - clean the MAF!

Well said! That is why I love these forums, they help clear up so many details and then others can reference them in the future.