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  #1  
Old 01-28-2017, 08:49 PM
Big Josh Big Josh is offline
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Default 4.0 OHV (misfiring/running rough)

Hi all,

I just picked up a 92 Ranger project/future daily driver. The truck has 142K (or 242K); the engine is original and looks clean. The truck has a 4.0 OHV engine with a manual transmission. It has been sitting for about 8 months, indoors I think. The truck is running now, but a bit rough/misfiring. There is spark. The engine maybe has one or two hours (10 miles) of running time since it started after sitting. As far as I can tell only the gas has been replaced along with all fluids being topped off.

I just did a compression test on a cold engine; a warm engine was not an option at the time. I had all the plugs out, pedal to the floor, and let it crank about 10 times per cylinder. Also the fuel pump relay was pulled and the coil was disconnected.

Results:
#1-185PSI
#2-190PSI
#3-165PSI
#4-160PSI
#5-190PSI
#6-190PSI
How are these compression numbers?

Cylinders 3 and 4 are 15+ percent lower than the others. I wish they were a little tighter. One of the valve cover gaskets (driver side) is leaking toward the rear. I was planning to replace both gaskets and adjust/examine the valves. Will this help with compression?
My concern is, that the heads have cracked water jackets. I read about this problem on another thread. (Symptoms + compression test results =??)
Nothing points to bad head gaskets/signs of cracked heads. The coolants clean and so is the oil. I do not know if it is losing coolant though.

Before I decided to do a compression test my plan was to change all fluids, new plugs/wires, clean injectors, and start cruising. I might be ok with replacing the heads, but ideally I didn’t want to do any major engine work for a few years (25K or so). I want to use the money I have on all the other things the truck needs. Remember it is a project!
Should I replace both the heads upfront instead of doing a bunch of work that I may have to re-do? What other tests should I consider to better pinpoint an engine problem?

I think the misfire is because of clogged fuel injectors, is this a common problem in vehicles that have been sitting for some time?

Any thoughts or recommendations will be appreciated.

THANKS in advance for all the help!
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Last edited by Big Josh; 01-29-2017 at 12:42 AM.
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  #2  
Old 01-28-2017, 08:51 PM
louisvilleranger95 louisvilleranger95 is offline
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Default Re: 4.0 OHV (misfiring/running rough)

Those are great numbers

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  #3  
Old 01-29-2017, 07:09 AM
whiskeymike whiskeymike is offline
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Default Re: 4.0 OHV (misfiring/running rough)

10% is generally the max you want between highest & lowest. 160-190 is a big jump.

You really need to do a warm test. Once you do that, if 3&4 are still low, put a little oil in them & re test. If they jump up, you have bad rings. If they remain low, it's in the valves somewhere.
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Last edited by whiskeymike; 01-29-2017 at 07:13 AM.
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Old 01-29-2017, 01:07 PM
Big Josh Big Josh is offline
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Default Re: 4.0 OHV (misfiring/running rough)

I agree that I need to do a warm test. I will work on that today and hopefully post the results tonight.

If the compression results are similar is there anything I can do to put off major engine work?
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Diagnose the problem! Don't make a wish and throw parts at it like I use too.
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  #5  
Old 01-29-2017, 01:15 PM
cowboybilly9mile cowboybilly9mile is offline
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Default Re: 4.0 OHV (misfiring/running rough)

Time has shown me that some people say 10% is acceptable, others say 20%; you get to decide what you think it right. What I've noticed in the real world is that what happens is, a cylinder(s) can get weak enough to cause a rough idle, next step is a progression to running rough at all times, and on OBDII vehicles, eventually it/they can trigger a DTC that is for misfire. In your case, IMO, I'd roll it and not drop a dime on heads unless an actual need is shown at this time. And while it is true that a warm engine does make a difference, I'd expect about the same overall results with just a slight, MOL uniform increase percentage increase on them. BTW, congratulations on pulling all the plugs and having a WOT while doing the test, I'm so tired of people that wonder why their compression is low and then it's realized that they didn't realize how to do it........correctly. +5!
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Last edited by cowboybilly9mile; 01-29-2017 at 01:19 PM.
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  #6  
Old 01-29-2017, 02:09 PM
Big Josh Big Josh is offline
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Default Re: 4.0 OHV (misfiring/running rough)

I think I will put off the warm test until I do a tune up and get this thing firing correctly. I really do not want to run the engine to much until I replace all the filters and liquids.

Thank you all for your help and suggestions. I'm with cowboybilly9mile on the fact that some say 10% while others say 20%. I wish there were better tolerance guidelines out there. For now I will start with the basics!

I will be back with an update and a warm compression test in a couple weeks. In the meantime, does anyone have any thoughts on clogged fuel injectors causing a misfire after sitting for 6 months?
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4.0 OHV 6 Banger

Diagnose the problem! Don't make a wish and throw parts at it like I use too.
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  #7  
Old 01-29-2017, 02:33 PM
cowboybilly9mile cowboybilly9mile is offline
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Default Re: 4.0 OHV (misfiring/running rough)

How do you know the cause of the misfire is an injector problem?
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  #8  
Old 01-29-2017, 03:06 PM
whiskeymike whiskeymike is offline
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Default Re: 4.0 OHV (misfiring/running rough)

Do like you wanted with the plugs and wires. Older ford injectors are pretty primitive, they don't tend to clog as easy. Even though sitting with today's gas can't be good, I would do plugs and wires before I went any further.

I would look at 20% as able to run, 10% able to run smooth/good. Even your lowest isn't that low, it's just the balance. At 15%, assuming your numbers don't improve I would expect it to run a little less than smooth, but not necessarily rough. I wouldn't rebuild it yet personally. Even if you decide it does need built you'll need plugs, wires, etc. for the new motor so you're not going to be out anything.
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  #9  
Old 01-29-2017, 08:50 PM
Big Josh Big Josh is offline
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Default Re: 4.0 OHV (misfiring/running rough)

Here is a little more of the back story:

When I bought the truck it was missing on #3; I know this because when I pulled the #3 wire the engine ran the same. All the other wires when pulled, caused a change. My concern at that time was that it had a dead cylinder, but I felt it was worth the risk and if need be I would do a compression test.

The first thing I did was check #3 for spark and found a fouled plug. I bought a plug and once installed the misfire was gone. The truck was firing on all 6 cylinders!

I took it for a drive around the block and let it run for 15min. Once it warmed up it seemed to be running descent (for a truck that obviously needs a tune-up). I turned it off while I worked on some other stuff and when I started it back up (cold) it was missing again. But this time #5 was missing. So I pulled the #5 plug, and it was caked up, but after I cleaned it up it had good spark. When I put it back into #5 it made no difference and #5 was still missing. I also switched wires and that made no difference. There seems to be spark on all 6 cylinders. This is why I did a compression test. (#5-190PSI/#3-165PSI) I needed to remove the plugs anyway.

The reason I suspect a clogged injector is because the plugs I pulled all had good spark. The wires had spark too. (They need replacing, but all seem to be functioning, so that makes me think its not missing because of spark). The compression test shows me that the misfire is not due to a compression issue (every cylinder should fire). Also the truck has been sitting for some time (6 months probable more) and the guy I bought it from told me he drained the gas and put a 1/4 tank of 91 octane in the truck. (Only a 1/4 tank and of gas that I think could be to rich) That doesn't seem like enough gas after that amount of time sitting. I plan to listen to the injectors when I re-start the truck if it still misfiring. Then I will know more, but was wondering was a possible or even common cause for a misfire.
*That reminds me I need to replace the fuel filter ASAP. Any recommendations on a good filter?

whiskeymike's post seems logical to me; start with the basics and work down the line gathering more evidence. Along with plugs(Motorcraft platinium) and wires; I think I will change the oil and replace the fuel and air filters. Then fill the truck up with regular and fire her up

Any recommendations on fluid choices? Additives?
Other things I should consider doing before the re-start? I was thinking of fogging the cylinders before I put the plugs in. Any thoughts on that?

The air intake tube is leaking were it attaches to the throttle body. This causes a check engine light, but when pressed in and sealed the engine light goes away. Either way there is no change in how the engine runs sealed or unsealed.
I plan to fix this, but could that cause a steady misfire on one cylinder only?Should I be looking into an air intake problem?

Again thanks to all! I am grateful for the input.
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92 Ranger 2WD Supercab
4.0 OHV 6 Banger

Diagnose the problem! Don't make a wish and throw parts at it like I use too.
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  #10  
Old 01-30-2017, 05:36 AM
whiskeymike whiskeymike is offline
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Default Re: 4.0 OHV (misfiring/running rough)

Sometimes you'll have good spark, then under pressure spark isn't as good. And anything but a bright blue spark isn't good enough.

The air leak can make it run rough because your getting airflow that isn't measured by the MAF. Generally it is more random than 1 cylinder, but it definitely needs fixed ASAP.

I use wix or Motorcraft filters.
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