Intermittent miss, air fuel won't richen under boost - Ranger-Forums - The Ultimate Ford Ranger Resource


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Old 10-24-2009
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Intermittent miss, air fuel won't richen under boost

I'm having a terrible problem with my new (to me) rig. I've got a 1994 Ranger with a 1997 explorer 5.0 running a small turbo @ 5psi. Truck was running good for most of the summer, but last month or so I've been getting a pretty hefty miss while maintaining highway speeds (I think it may be there at low speed too, just not very noticeable), doesn't seem to miss while accellerating or coasting. The truck was also tuned (Sniper Special Forces) to run richer when getting into boost, it no longer does this (alwayse tries to maintain its cruising a/f of 14.7-15.1 on the A/F gauge).

I've changed a fuel pump (walbro gs341 255lph high pressure), fuel filter, fixed a ruined EGR hose to the DPFE, and new spark plugs. It had 3 codes thrown (no CEL), I believe 2 of which are hold overs from the swap having to do with automatic transmission, and a VSS code, also probably due to no anti lock brakes from the swap(?). No CEL after clearing, going to try and read again monday or tuesday.

Still has miss and A/F problem, and I'm running out of ideas. Can anybody point me in the right direction? I'm afraid of a PCM problem, as I no longer have access to the Sniper Tuner and can't reflash, but how could I check that? And how could it still run so good under most conditions?

Some specs; 1997 expo 5.0 (gt40p I believe) has 39# injectors, 90mm lightning MAF, remote To4e turbo @ 5psi, Walbro 255lph hi-pressure fuel pump, m5od-R2 and 1356 manual trans, AEM a/f and autometer vacuum gauge.

Any ideas would be greatly appreciated, I don't trust my dealership to know much about a rig like this and I can't afford them anyway. Thanks in advance.
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Old 10-24-2009
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Is the miss at all related to any RPM range?
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Old 10-24-2009
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Well, I don't think so, but I notice it most around 2000rpm or so (65mph or so). But I believe it is there at other RPM's too, but I'm fairly sure it doesn't do it when you start getting on the gas pedal (at any RPM), and I haven't noticed it while coasting (at any rpm) and definitely not at idle (idles very well).
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Old 10-25-2009
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As long as your A/F doesnt go leaner than 14.7 under boost you are fine. there is noneed to dump extra fuel.
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Old 10-25-2009
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Well, I was just wondering, under a load and at lower RPMs, a bad set of plug wires will cause a miss like you described.
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Old 10-25-2009
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Check your maf sensor and fuel injectors for dirt

Also your spark plug wires
just my .02 cents
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Old 10-25-2009
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I suppose it could be plug wires, they are 8mm motor craft, but I suppose they are closing on 3 or 4 years old. I've been trying to pay more attention to RPM ranges, and it definitely seems to be around that 2000rpm (more specifically 1600-2100 under very light acceleration or maintaining), haven't been able to replicate it at 3000+rpm yet, and idles very well.

I also keep coming back to DPFE sensor in my own mind. Is it stuck in some oddball position that is messing up my computer and keeping it in a low power fuel table, and trying unsuccessfully to recycle at 'cruise'?

I realize I'm running fairly minimal boost pressure, but from what I read its fairly important to run rich (11.5-12) when pulling in boost to avoid catastrophic detonation.

Thanks for the help so far folks, its good to have help.
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Old 10-26-2009
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Quote:
Originally Posted by Grot View Post
As long as your A/F doesnt go leaner than 14.7 under boost you are fine. there is noneed to dump extra fuel.
Run away from advice like this..




You A/F ought to be in the mid to high 12s when under boost. If you ever see a 13.5 or higher... get out of it!

It sounds to me like what ever is mechanicly connected to the fuel pressure controler is not working properly.

Tunes don't change once flashed. However plug in chips can come loose. Which do you have?

The miss... could be a vacuum leak, a coil-wire-plug, or maybe a foulty injector.

Rich
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Old 10-26-2009
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Quote:
Originally Posted by Lignite View Post
I realize I'm running fairly minimal boost pressure, but from what I read its fairly important to run rich (11.5-12) when pulling in boost to avoid catastrophic detonation.
Generally speaking a high 11 is plenty fat until you start to push over 20psi and/or put nitrous on top of it.

With my lightning I'm pushing 18psi with a eaton (higher inlet temps than a cooler turbo) and I'm commanding a 11.6A/F (approx 550hp at the crank)

Rich
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Old 10-27-2009
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Thanks for the confirmation Rich, I want to keep this rig as reliable as possible. It is a homebrew tune, built and flashed to the PCM with Sniper Special Forces (which I no longer have access to). If the PCM was heading south I should see lots of other problems surfacing, correct?

I guess the advice I'm seeing most is plugs and/or coils, and while I can't see how that can affect my a/f ratio, I can see where they could cause a miss. I guess the next step is to make few calls and find some good wires. Don't suppose anybody can confirm the 4cyl Screamin Demon coil (two) will work on the 5.0?
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Old 10-27-2009
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Well, if you can confirm that your A/F is not rich enough, that could cause a miss too.
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Old 10-27-2009
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A/f seems to be good when it misses, according to AEM wideband a/f gauge, but jumps lean for brief second after the miss. Would love to have a real diagnostic log, but that may have to wait for Christmas.
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Old 10-28-2009
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Just a thought, but if its going lean when you get the miss, it sounds to me more like an injector since you would be adding less fuel to the mix, where as with a spark issue, you would be running rich from the unburned fuel heading out the exhaust. It could also be a fuel related issue. You may want to check your fuel pressure off of the fuel rail to make sure your getting enough pressure to feed those big injectors.
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  #14  
Old 10-28-2009
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That does make sense, and I've intended to check fuel pressure (had that good advice quite a bit too) but haven't got it done yet. I changed out the fuel pump early as it seemed very similar to a problem I had on another Ranger (3.0l), but that was a fail. I haven't figured out exactly 97 5.0 fuel system works yet, but I believe it is a return style, with regulator in the tank. If it was bad it wouldn't run right at all, I would think. Could be fuel injector though, yuck, how to trace that down? hopefully check fuel pressure and codes thursday or friday. Going to hold off on plug wires and coil for right now (none at my local parts dealer anyway, and hate to go to the stealership).
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Old 10-29-2009
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Quote:
Originally Posted by Grot View Post
As long as your A/F doesnt go leaner than 14.7 under boost you are fine. there is noneed to dump extra fuel.
This person has no clue on tuning, yet alone turbocharged vehicles.

I ran a solid 11.7 to redline on my 93 octane map, and 13.5 with methanol injection. I tune turbocharged engines producing 2x, 3x the stock engine torque and power values regularly.

I would change plug types, or ignition and see if it goes away.

You really need to log your afr and timing if you have these capabilities. You should also be logging boost with a map.

Try logging with the wastegate pinned open and see how that rides first - have you done a boost leak test? have your injectors been properly scaled? Do you have excessive backpressure? (are the converters plugged?)

Last edited by Jp7; 10-29-2009 at 02:29 AM.
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Old 10-29-2009
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Quote:
You A/F ought to be in the mid to high 12s when under boost.
This is very high in my opinion, even though I do know you know what your talking about.
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Old 10-29-2009
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WOW....stop the insanity. High 12's under boost??? Damn.

Dude, I don't know if you realize that in the fall/winter we get a different blend of gasoline. There is more methanol/ethanol in those blends than the summer blends. It might just be as simple as going a little richer on your tune. I would also check if your fuel pressure is going up as your boost is going up. Also get a set of Ford Motorsports 9mm wires. Also, what is your spark plug gap at?? I would start at .030 and go from there. Do the small stuff first.
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Old 10-29-2009
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Originally Posted by Red91Stang View Post
WOW....stop the insanity. High 12's under boost??? Damn.

Dude, I don't know if you realize that in the fall/winter we get a different blend of gasoline. There is more methanol/ethanol in those blends than the summer blends. It might just be as simple as going a little richer on your tune. I would also check if your fuel pressure is going up as your boost is going up. Also get a set of Ford Motorsports 9mm wires. Also, what is your spark plug gap at?? I would start at .030 and go from there. Do the small stuff first.
good advice.

for example i run e85 (i can, even know I prefer methanol), sometimes it is like e70 in the winter
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Old 10-29-2009
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new copper plugs are gapped at .052 (stock I believe), old ones were worn out to about .060. I was under the impression that at these low boost levels (5psi currently) this gap was acceptable?

This tune was set up and running quite good, with a pretty solid 11.7 when you put the hammer down, but it no longer does that. A/f hangs at 14.6-15.1 which is the low power fuel table. But I can't seem to figure out why it won't get into that richer fuel pattern when its supposed to.

I keep coming back to the EGR system. Due to the miss at highway speeds (when I think the EGR is trying to recycle exhaust?), I wonder if it is somehow stuck open (closed?) and is confusing the computer into not using the right fuel table at the right time? I'm so confused, I guess I just need to buckle down and replace some more parts.
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Old 10-29-2009
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EGR is just bad for power producing turbo setups. You should look into eliminating it.
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Old 10-29-2009
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I'm going to guess that the miss and the A/F ratio are probably not related. At least not to the same cause.

Do you have a fuel pressure guage in cab to see what the fuel pressure is doing under load? Is the system set up to ramp up fuel pressure with boost or was the increase in fuel done solely by the computer?

With plug gap, I was running .35 with my Powerdyne blower at 6 psi. I think .50 might be a little wide for a boosted motor. But if the spark was blowing out it would be at higher rpm/boost level in my expierence. I understand that blower motors and turbo motors behave a little differently but I think plug gap is kind of a common denominater between the two.

With the miss, I'd start looking around the plugs for things like cracked porcelin on the plugs, spark jumping from the plug/coil, etc. I'm thinking along those lines because of the sparatic miss. I'd suggest plug wires but this motor is a coil on plug set up, isn't it?

It's cool to see that there are some very knowledgable folks on this site!

Last edited by pinellas50; 10-30-2009 at 12:28 PM.
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  #22  
Old 10-29-2009
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I'm not mechanical enough to know what plug on coil means for sure, but I don't believe the '97 5.0 is, as it has plug wires and dual coil packs (no distributer). I ordered new plug wires and a new dpfe tonight. Will get them put on asap, and when those things don't cure it I'll be back begging for more help! Thanks very much for the help so far, this site has always been a wealth of information and help, and guys like me really appreciate it.
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  #23  
Old 10-30-2009
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plug on coil means it has no spark plug wires the coils are connected right to the plugs
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