2000 Mountaineer 4.0 SOHC P0174 backfires when in gear accelerating | Ford Explorer Forums - Serious Explorations

  • Register Today It's free!

2000 Mountaineer 4.0 SOHC P0174 backfires when in gear accelerating

January 22, 2020
Reaction score
Year, Model & Trim Level
00 mountaineer base
So I've read some other similar post and wanted to post mine as well to search further for solutions seeing as how I've yet to easily fix my own. The issues I have is that when I crank it (it cranks first time every time fine) the engine sounds fine at first. After putting it in gear the RPM lower as you would expect (although probably too much seeing as how it just idles at 600-650 RPM to boot) Then when I hit the accelerator there is a noticeable lack of power and if you go say over 2500 RPM the engine will start backfiring ( I assume through the intake because of the lean code and it sounds like it is coming from the front of the Vehicle) I'll list what I've replaced:

Transmission (unrelated I hope, the old one slipped)
Fuel pump (replaced strainer and internal hoses for the assembly as well)
Fuel filter
spark plugs (platinums)
spark plug wires (golds $50)
Air filter
EGR valve
A single injector (for now and replaced O-rings for all injectors on bank 2)
Upper and Lower intake manifold gaskets
A single O2 sensor so far

Filled the gas tank then emptied a can of seafoam injector cleaner into the tank.
Hot soaked the engine with a can of seafoam after getting it to temp. for 10 minutes then ran it afterwards
I've cleaned the MAF sensor with MAF sensor cleaner
Cleaned Idle Air control with the same cleaner
cleaned butterfly valve with carb and choke cleaner

Fuel pressure at rail: 60 PSI (only at idle and revved in park because the hose isnt long enough for me to see while driving)

throws misfire codes for bank 2 when I take it on the road and accelerate in gear.
It seems to idle at about 600-650 RPM

I did see a fuel trim freeze-frame readout. You will notice long term on bank 2 is 19%.

I made a vacuum smoke tester... out of a soldering Iron in a glass gar (epoxied the iron facing down through the lid where the tip would burn socks soaked in mineral oil inside... then just attached air into one side of the top of the lid and ran a hose out of the other side of the top of the lid.) The way I tested was to remove the breather hose from the breather box, then covered the opening of the hose with a rubber latex glove. I then removed the brake booster hose from the brake booster and inserted the hose from my smoke tester and ran smoke through that way.

At first there were a few areas that leaked smoke and I addressed them. the hose from the breather hose that goes to bank 1 head was replaced because of a leak. I cant recall anywhere else that I could see leaking the smoke. I'm still under the impression that I could still have a leak due to the engine only really acting up when i'm accelerating in gear. I put maybe 20-30 psi of smoke through it and it held that fine but i'm not sure if there's a lot more pressure or vacuum when the engine is under a load. Any info on that would be greatly appreciated.

I have removed the exhaust and inspected all 4 catalytic converters. The first bank 2 converter is or at least has been clogged because the honeycomb was melted looking and a good bit of it broke up and fell to the first big converter. In this pic just ignore the mirror piece I was previously trying to use it to look inside until I got an endoscope. I actually drove the truck without the exhaust after the manifold... It still backfired and skipped under acceleration so im not sure how much better it will run after spending $250 on this dual converter y-pipe.

Basically I'm not an expert or even a seasoned shade tree mechanic for that matter... I just bought this truck from my mom to help her out and figured I'd fix it up in the meantime but it's really giving me a headache at this point. If there is any more info I can provide just let me know ill keep an eye on this thread and just any insight to the next thing I should try is greatly appreciated. Im not sure like I said earlier if I'm having so much trouble diagnosing this issue because it only acts up under a load and just cant replicate the problem while being under the hood or perhaps I just needs some fresh eyes on the problem. Thanks.









Join the Elite Explorers for $20 each year.
Elite Explorer members see no advertisements, no banner ads, no double underlined links,.
Add an avatar, upload photo attachments, and more!

Running lean will cause backfires (unburned fuel enters the exhaust and is ignited). Sounds like a timing issue. The SOHCs had a camshaft position sensor that drives the timing. It might have gone bad.

Or you may have jumped a tooth on the actual cam... do you have timing chain rattle?

There's no rattle at all. The truck actually runs pretty good in park. you can take it up to 5000 to 6000 rpm and it still purr like a kitten. Only when the engine is under a load by being in gear that it starts complaining. Also another thing to note is when I got it all there was for any codes was an occasional misfire code for cylinder 5 only. I then replaced just that plug at the time and it didnt help. After that I tried an injector for that cylinder...which involved me removing the upper intake, unplugging the computer, and remove all the vacuum lines from that upper intake. I believe it was after installing that injector that I started having this backfiring issue and skipping while accelerating in gear.

Usually its better to replace the injectors in sets, they are relatively inexpensive. Also it may be the wrong injector or it may be stuck open. Coil pack is also another place to look it might be generating a weak spark. Combined with a new injector that may be flowing more fuel may cause the truck to backfire.

Ok. I agree replacing them in sets is better. Ill do that as well just might take me about 2 weeks to get the spare money up to do so... they are wanting like $60 per injector in the places i've looked at so far and found a couple of sets at around $160. In the meantime I think i'm going to put the intake back together and try to get some live data with a bluedriver obd II scanner I bought the other day. Freeze frame is ok but maybe if I can isolate some thing out (like O2s switching voltage properly and what-not). I'd also like to try and reset the PCM (hopefully just leaving the battery disconnected overnight will do it) and see how the fuel trims react after being reset. Anything I found out ill post.

On the injector you pulled off, on the side should be a long number. Use that number to find your injector (they are all Bosch anyway). There were some injector changes (Flex Fuel vs non Flex Fuel) over the years and you may have gotten the wrong one.

Have you done a compression check? If a cylinder is low in compression it could be a warped head, bad rings, cracked or worn valves. If you find a low cylinder squirt a little oil into spark plug hole and retest. If pressure goes up you have bad rings. If pressure stays the same you have bad valves or warped head. Either of those could let un-burned fuel into exhaust (if it is the exhaust valve) and cause back fire. If the intake valve is not seated right and when cylinder fires the valve will let some of the exploding gases back into intake.

So I finally got around the weather to getting the exhaust back on and getting some live data. I did originally do a compression test just on cylinder 5 (it was the only misfire code at the time and its hard to get that tester is the plug holes lol) and I got around 150 PSI so I can at least confirm that cylinder is good. I'm new at trying to understand what this live data is telling me so if anyone else is better at gaining any useful information with this that would be awesome. I will note that it is strange that data is supported for sensor 1 and 2 on each bank when I am fairly certain there is only 3 O2 sensors in total (1 for each bank right after the exhaust manifold at which point the pipes "y" together and then there is 1 O2 sensor after that in between 2 cats). Also note that I took these screenshots just overtime as I cruised around town...so you can assume that the graphs with oscillations in the data is just data gathered later in the test drive (due to just having more data) so that should give you a sort of time line. I can say that on the o2 sensor 2 bank 2 and bank 1 sensor 2 the only major changes in those graphs occurred when I accelerated harder.











View attachment 177786

You are running pretty lean on bank 2 . Should be +/-5% on the fuel trim. (bank 1 is passenger side, bank 2, driver's)

Lean is too much air not enough fuel. The computer is overcompensating by adding more fuel in order to maintain the A/F ratio and since it can't, excess fuel is being dumped into the exhaust and exploding there, thus your backfire and why your cats are bad and why that O2 sensor needed to be replaced. (BTW they are best replaced in pairs)

Exhaust header/collector leaks can cause lean conditions (wrong information getting received by O2 sensor), but the fact that you are running lean on one bank vs the other means its probably not a vacuum issue (unless the lower intake gasket was not seated correctly when installed on the driver's side). This would cause both banks to be lean.

Was this data pulled during closed or open loop operation?

Other possibility, and this is out there, is that you have a head gasket leak. Do a compression test on 4 and 6 as well, very low compression on one of those cylinders may confirm this. Cylinder leak down test might as well. Also, these heads are aluminum so they really don't crack, but if overheated they can warp. Not saying yours has but it's something to keep in mind. Not all head gasket leaks involve white smoke and coolant loss.

I am almost certain it was ran in closed loop operation. I can never be %100 though because my memory is getting bad. I've take the upper and lower intake off of the truck four times now and did so carefully So I believe the gaskets to be good (replaced them the last time I took it apart). I will do the compression test on those other two cylinders and look into the exhaust header/collector soon and get back with you. I don't have the equipment to perform a leak down test so if that becomes more necessary I'll make that happen later. Another thing I was noticing while running these tests is that it seemed like most of the time when the fuel trim for bank 2 was way too high, the fuel trim was the same percentage too low. It only happened every so often so I only got a screenshot of it only moderately happening. It could be just happenstance but a few places suggested that the O2 harness could have been switched when I replaced the transmission. I don't believe that is the case though just because the cat is blown out on bank 2 just as the data suggests. At any rate thanks for the help so far again.

I am back with some cylinder compression readings. The compression came back pretty good but ill post pics of the data I got. I only tested bank 2. The pictures in order from top to bottom are back, front, then middle cylinder compression readings then the same for spark plugs (the first and second plug looked pretty much the same and I forgot to take a pic of the first plug so you are actually seeing the same plug for the first 2 plug pictures). Also worth mentioning these plugs are new.... probably don't have 20 miles on them yet. Also did spray some soapy water on the top side of the exhaust manifold where it meets the block and started the truck. wasn't any noticeable leak that I could detect.







Yup cylinder 5 is getting fouled with too much fuel. Now I'm starting to think coil pack. Check the plug on cylinder three. Coil pack fires in batches of two, 1-4, 2-6, 3-5.

here is cylinder 3. All of the plugs I pulled were covered in gas. although plug 5 was fouled and black. I just assumed it was from the ecm overcompensating the gas for bank 2 but plug 3 is also covered pretty heavily in gas and its in bank 1... Strange especially since I had the fuel system disabled when testing for compression and still had spark... to my knowledge at least.


Me thinks you have a weak spark or no spark from the coil on those cylinders. 3-5 are both fired at the same time by the coil. Since the explorer uses a waste spark system the coil is fired in pairs, one on the exhaust stroke and one on the compression stroke. It made things simpler for the electronic ignition. That's also why you only have 3 yellow wires (plus the the red power and black ground wires) going into the coil pack, each wire fires a pair.

Test voltages on the coil pack, you should see 12 volts on the red wire. If you don't then it is a power issue, otherwise you may need to replace that coil pack.

Also pull the camshaft positions sensor readings and compare them with the PIP signal. You should roughly see for every two rotations of the crank, one turn on the CMPS. If that signal is off it will fire the injectors at the wrong time which also can cause backfiring.

And check the coolant temperature sensor readings. If the CTS is bad it can also cause issues.

So far I've managed to check the voltage on the coil. while running Im getting 13.8v to the red wire. I can check the coolant temp sensor reading with my code scanner and it seems well enough, however my scanner doesnt seem to support any kind of fuel injection timing diagnostics. So now Im kind of stuck figuring how to get cam sensor reading and checking PIP.

Time to get forscan. It costs nothing and installs right on your laptop. All you need is the right interface cable (don't go with anything other than the recommended one on that page) and then you can start pulling live data like a pro.

This is what mine was pulling when I was diagnosing lean codes on my 98 V8. PIP is the green line and the CMPS is the purple. I was also able to track misfires (red line), look at spark advance (magenta), short term fuel trim on both banks (yellow and dark purple) and RPM (cyan). May be a little different for a SOHC.


I will absolutely look into this thanks. Its been kind of tight around here lately so haven't been able to invest anything into the truck (tax time and probably going to have to pay them dearly) but within the next few weeks I should be able to start knocking a few things out. I did get a few codes while trying to run the scanner which was a o2 sensor reading too high on bank 2 sensor 1.... not sure if it was because of a cold start. but also cylinder 5 and 6 misfire. So first thing ill replace is that o2 so they will have been replaced as a pair.... I really should do the rest of the injectors as well (not looking forward to taking off the upper plenum a 5th time). if nothing there I will get this forscan cable and run a few more tests... if the fuel injection timing is good ill probably at that point replace that partially blown out catalytic converter and see where that gets me. If its not those issues Im just going pray that a plane crashes into my house lol... I don't want to deal with the 500 timing chains inside this thing.

I wasn't going to mention timing chains...

...yet :shifty:


Join the Elite Explorers for $20 each year.
Elite Explorer members see no advertisements, no banner ads, no double underlined links,.
Add an avatar, upload photo attachments, and more!

Backfiring is usually timing related. Spark plug firing with a valve open at the wrong time. On a Ford 4.0L SOHC that means timing chains.