Replace Mopar MPFI with Carb

[timo439]

All,
I have been dealing with the dreaded MPFI ping for awhile to the point where I dont like to drive on the highway because the revs cause a constant ping. I do run 93 octane and the engine was rebuilt 6000 miles ago so not much left to try. I am at the point where I am seriously contemplating migrating back to a carb set-up. Its that or go whole hog and replace the existing head with a 4.0 head. Given the scrambler gets driven ~3,000 miles each year, dropping the couple grand on the head replacement seems extreme. Thoughts on my line of thinking? For someone close to NH, I'd be willing to trade the entire MPFI set-up for a carb set-up if you could do the install.

Tim

 

[mysunnshine]

You just need to relocate the crank sensor a little to retard the timing.

Sent from my SM-G920V

 

[spankrjs]

I will gladly mail you all of the stock stuff off my green '83 for your Mopar set up

In all seriousness, my red Scrambler with MPI has never pinged, no matter the altitude or octane. The timing might be an issue, the instructions were pretty clear about making sure the distributer was dropped in at TDC, worth a shot

 

[timo439]

Just bought a timing light. If I can figure it out, at least I'll know if timing is set right
thanks for the advice.

 

[timo439]

First time with the timing light and if I read this correctly, I am way off... I have the video, just need to get it loaded somewhere.

 

[timo439]

Video

[video=youtube_share;mSNZOEBsZb0]http://youtu.be/mSNZOEBsZb0[/video]

 

[spankrjs]

IIRC, don't try to change the timing with the distributer.

Your best bet:

1) Pull the distributer out
2) Pull all the spark plugs
3) Turn the engine over by hand until you get #1 TDC
4) Reinstall the distributer

IIRC, the distributer only fires the plugs. You can fine tune the timing with small distributer adjustments. If it is way out, TDC the engine and start over.

Check the clearance between the crank position sensor and the harmonic balancer. Also, check for a slipped harmonic balancer.

 

[mysunnshine]

The crank sensor location controls the timing. By moving the mount clockwise, you retard the ignition timing. When you move the distributor, you change the cam sensor position which changes the injector opening and closing time(degrees). You also change the rotor phasing(which is rotor to cap post location) by turning the distributor body. If you have knock, you need to move the crank sensor a little bit clockwise. That will pull some total timing away.

Sent from my SM-G920V

 

[trust]

Thank you sunnshine, ill be trying it tonight if its not raining in the am if it is

 

[LetchcoreCJ7]

I didn't know you could even adjust the timing at all on a mopar mpi kit. I just always set it at zero and thought the computer did the rest. I never had any pinging so maybe I never needed to adjust .

 

[mysunnshine]

Thank you sunnshine, ill be trying it tonight if its not raining in the am if it is

Make sure you document your timing before you make any changes so you know how much of a chage you made(advancing or retarding) in the timing.

 

[CJ-8_Jim]

I have the Hesco MPI and had the ping. I got rid of the ping with an adjustable fuel pressure regulator.

 

[trust]

While i havent finished relocating the CPS yet i can say that initial timimg at idle as is is about 18* btdc.

Once i get back on this issue and can drive it again ill share where its at, what i did and whether it helped

 

[spankrjs]

The crank sensor location controls the timing. By moving the mount clockwise, you retard the ignition timing. When you move the distributor, you change the cam sensor position which changes the injector opening and closing time(degrees). You also change the rotor phasing(which is rotor to cap post location) by turning the distributor body. If you have knock, you need to move the crank sensor a little bit clockwise. That will pull some total timing away.

Sent from my SM-G920V

Quick question - Does a factory 4.0/MPI set up use a knock sensor to retard timing?

The stock 4.2, with the feedback carb system, uses a knock sensor on the intake manifold to retard the timing if pinging/knock occurs.

My GM TBI uses a knock sensor/ESCM to retard timing if necessary.

How does a stock 4.0 electronically control spark advance

I am running the Hesco/Mopar MPI on my red scrambler. I installed the distributor at TDC per the instructions, I have never had "pinging" I can check the timing to see what it has at idle, if that will help

 

[CJ-8_Jim]

All,
Its that or go whole hog and replace the existing head with a 4.0 head.
Tim

FWIW, the 4.0 head didn't solve my MPI ping. Only the adjustable fuel pressure regulator that allowed me to increase pressure... i think I had to increase it only a couple psi.

 

[trust]

I have the 4.0 head and ping, i guess after the timing experiment comes the pressure reg. My system has no return line, does that matter?

Id be interested in your system timing Spanks if u get a chance

 

[spankrjs]

I have the 4.0 head and ping, i guess after the timing experiment comes the pressure reg. My system has no return line, does that matter?

Id be interested in your system timing Spanks if u get a chance

No problem

My system has the return line.

 

[CJ-8_Jim]

You just need to relocate the crank sensor a little to retard the timing.

Before fooling with the the CPS, I would do a couple other things first (that is #1 and #2 below and possibly #3).

When the OP says he has pinging, I'm assuming he is talking about detonation (after spark) and not pre-ignition (before spark).

There are 1 or more reasons that could be at play causing the "pinging." The thing to remember is the detonation heats up the engine faster and a hotter engine creates more detonation -- it is a vicious circle.

Also, the higher horsepower per cubic, the more sensitive the engine is to detonation. (remember, the OP says the engine was rebuilt./.. so that begs the question whether displacement was increased). If the engine was recently rebuilt, that may be aggravating the detonation condition.

Here are 3 things I would look at first before investing time in a new head or removing MPI altogether (as the OP stated).

Generally, running too lean an engine will cause increased temperatures. Therefore, i would start by checking or experimenting (I say "experimenting" since a rebuilt engine is not OEM-spec any longer and therefore original requirements may no longer be valid):

1) Is the correct spark plug being used. OEM for the a 95 YJ MPI is a champion RC12*, but HESCO recommends a colder plug RCJ9*. Also note, that since the engine was rebuilt, the displacement might also impact the heat rating of the desired spark plug, so the heat rating might not be a 12 nor a 9 but something else.

2) Fuel pressure. Lower the fuel pressure, the leaner the air/fuel mixture and hotter the burn. When I put an adjustable fuel pressure regulator on mine and increased the fuel pressure, I solved my pinging problem. My block was recently reman, so my displacement may have increased (who knows) aggravating the tendency to ping.

3) Injector size. More displacement, needs more fuel. Did the engine "rebuild" increase displacement? Where the cylinders bored out?
Trying different injectors is a very expensive "experiment", but if the guy who "rebuilt" the engine says he increased displacement/stroked it, it might be worth exploring the possibility that bigger injectors are needed.

Just my 2 cents.

Jim

 

[CJ-8_Jim]

P.S. I my memory serves me correctly (and I could very well be wrong), I seem to remember that in theory, removing the vacuum from the fuel pressure regulator will increase the fuel pressure.

Therefore, before doing anything else (even before doing my #1 or #2 above), I'd be curious how the engine ran with the fuel pressure regulator vacuum line disconnected (note: when doing this, you may have to plug the vacuum line).

I've never done this and I'm not qualified to say what the downside risks are to doing it, but I can be adventurous at times and it seems like it would be worth momentarily "experimenting".

P.S.S. FWIW, Retarding spark (assuming the spark is set at OEM spec) will reduce torque.

 

[mysunnshine]

If you disconnect the vacuum line to the fuel pressure regulator, you will increase the fuel pressure everywhere but WOT. The higher the vacuum, the more responsive the FPR will be if connected. Your fuel trims may also limit out because it's going to need to pull the "extra" fuel out that will be added if the vacuum is disconnected.


Yes, if you retard the timing, you can decrease the engine torque assuming you have the correct amount already. If you are already over advanced and detonating, then you have an extreme torque loss now. Granted, by taking out a global amount of timing by moving the crank sensor to stop mid to higher load pinging, you may lose some top end torque. That's really hard to tell unless it's on a dyno. Unfortunately there is no access to the software to custom tune the timing and fuel maps.

Sent from my SM-G920V