Am I Diagnosing This Misfire Correctly?
Thread Starter
Joined: Nov 2007
Posts: 413
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From: Albuquerque, New Mexico
Am I Diagnosing This Misfire Correctly? So I got a misfire code and checked it out using the old school method of spraying water on the header tubes. The misfire is not random. It is specific to the 6 and 5 cylinders. Has to be a coil, spark plug or wire....right?
A little history....The long block has just 9500 miles since new. The coil pack is the older style. It was a "used" purchase. The spark plugs and wires were new. The fuel injectors were used but I had them cleaned and flowed. I had the gas tank flushed, a new fuel pump and sock installed. The gas filter was replaced. The truck intake was cleaned. Everything ran great except during it's only summer. The truck stalls on hot days and I'm pretty sure it's because the Granatelli MAF sensor is throwing very sporadic rich codes. A new stock truck MAF has been ordered. I'm thinking the rich mixture has finally fouled those two plugs and my older coil pack can't compensate.
A little history....The long block has just 9500 miles since new. The coil pack is the older style. It was a "used" purchase. The spark plugs and wires were new. The fuel injectors were used but I had them cleaned and flowed. I had the gas tank flushed, a new fuel pump and sock installed. The gas filter was replaced. The truck intake was cleaned. Everything ran great except during it's only summer. The truck stalls on hot days and I'm pretty sure it's because the Granatelli MAF sensor is throwing very sporadic rich codes. A new stock truck MAF has been ordered. I'm thinking the rich mixture has finally fouled those two plugs and my older coil pack can't compensate.
easiest way to diagnose a misfire is one by one swap an injector with a known working one, swap a coil, swap, a wire replace all plugs. if the misfire moves when you swap one part you found your problem
To your list you can add intake runner gasket leaks,
and fuel injector delivery (imbalance) both of which
could make one hole go lean.
I've seen people say that the misfire cylinder number
is not reliable, can be off from the real culprit position.
So if you can't find a cause, may try swapping to the
left and right and look for the reported misfire to move,
no matter what cylinder it says it's about. Keep your
eye on the thing that's being swapped, and look for
whether it made a difference - expected position or not.
and fuel injector delivery (imbalance) both of which
could make one hole go lean.
I've seen people say that the misfire cylinder number
is not reliable, can be off from the real culprit position.
So if you can't find a cause, may try swapping to the
left and right and look for the reported misfire to move,
no matter what cylinder it says it's about. Keep your
eye on the thing that's being swapped, and look for
whether it made a difference - expected position or not.
Thread Starter
Joined: Nov 2007
Posts: 413
Likes: 0
From: Albuquerque, New Mexico
Great advice. Thanks.
I went ahead and purchased the upgraded truck coils with the heat sinks. It's an upgrade I've wanted to do for awhile. I had some rewards points through AutoZone so I bought their highest quality lifetime plug wires and some TR5 plugs. Didn't help. Replaced the Granatelli MAF with a stock truck. Did nothing but lose power. Guess those things work...LOL. Still chasing those two weak code throwing cylinders. Will check the intake gasket with starting fluid and change the two fuel injectors if need be. When I had the injectors cleaned 18 months ago, the shop said they had to run two of the injectors through the cleaning process twice to get them to work. Maybe those are the culprits.
I went ahead and purchased the upgraded truck coils with the heat sinks. It's an upgrade I've wanted to do for awhile. I had some rewards points through AutoZone so I bought their highest quality lifetime plug wires and some TR5 plugs. Didn't help. Replaced the Granatelli MAF with a stock truck. Did nothing but lose power. Guess those things work...LOL. Still chasing those two weak code throwing cylinders. Will check the intake gasket with starting fluid and change the two fuel injectors if need be. When I had the injectors cleaned 18 months ago, the shop said they had to run two of the injectors through the cleaning process twice to get them to work. Maybe those are the culprits.
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Verdad Gallardo Thread Starter
Joined: Nov 2007
Posts: 413
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From: Albuquerque, New Mexico
Now you have me thinking..... So the motor is a swap into a 1998 Ford F150. I tried to eliminate any issues by purchasing new parts as I performed the swap. I bought a fuel pump designed for a Lightning because it was a straight swap. It was a high volume pump and being a newbie to LS motors, it didn't occur to me that the Lightning doesn't require the higher fuel pressure that an LS motor does. I have been fooled by the high volume = higher pressure trap. I'm thinking (hoping) that the two weaker fuel injectors are now being affected by a drop in pressure as my fuel filter gets loaded. I have probably always was on the edge of not having enough fuel pressure because the Ford fuel system was not designed for it. But because everything was new, it got by. Thoughts??
If you have a return line on your rails, then fuel pressure will be less at an idle. Check it with the vacuum line to the regulator disconnected. 53lbs sounds about right for idle with the regulator connected to vacuum, 58 with it disconnected.
What do those 2 plugs look like compared to the others. Post a picture if you can.
What do those 2 plugs look like compared to the others. Post a picture if you can.
Last edited by Bowtie316; Jun 24, 2015 at 12:24 PM.
Thread Starter
Joined: Nov 2007
Posts: 413
Likes: 0
From: Albuquerque, New Mexico
If you have a return line on your rails, then fuel pressure will be less at an idle. Check it with the vacuum line to the regulator disconnected. 53lbs sounds about right for idle with the regulator connected to vacuum, 58 with it disconnected.
What do those 2 plugs look like compared to the others. Post a picture if you can.
What do those 2 plugs look like compared to the others. Post a picture if you can.
Now you have me thinking..... So the motor is a swap into a 1998 Ford F150. I tried to eliminate any issues by purchasing new parts as I performed the swap. I bought a fuel pump designed for a Lightning because it was a straight swap. It was a high volume pump and being a newbie to LS motors, it didn't occur to me that the Lightning doesn't require the higher fuel pressure that an LS motor does. I have been fooled by the high volume = higher pressure trap. I'm thinking (hoping) that the two weaker fuel injectors are now being affected by a drop in pressure as my fuel filter gets loaded. I have probably always was on the edge of not having enough fuel pressure because the Ford fuel system was not designed for it. But because everything was new, it got by. Thoughts??
I'll check with the vacuum line disconnected. Thanks for the lead. There was nothing different about the two plugs. They all looked somewhat fouled. I should have purchased the TR5s from the get go. The high dollar platinums with the tiny electrodes seem like a waste of money. I also gapped them incorrectly. Had about .010 too much. That was a year and a half ago so I can't tell you how I made that mistake.
If the plugs all look the same and you have 2 weak cylinders, it would generally indicate a mechanical problem. I would check compression on all cylinders. If that doesn't indicate a problem, I would move the injectors to different cylinders to see if the weak cylinders move.
Do a compression test, then a leak down test.Ii had cyl 7 completely dead triedeverything. Did a comp test had 0 the needle didn't even flick. Did a leak down had a wide open leak. Pulled the head had a pcv pipe clip holding myvalves open from my intake swap. Must've dropped it inthere by mistake. When you do a leak test listen to the throttle body, if you hear air you have a leaking intake valve listen to the exhaust if you hear air it's an exhaust valve problem.Listen to the pcv breather if you hear air it's a bottom end issue.
Also pull your valve cover check for broken valve springs or bent push rods
Also pull your valve cover check for broken valve springs or bent push rods
The Fords of the '80s / '90s all were 3-bar (42-ish PSI).
The GM scheme has a blowoff regulator on the return
and the pump just gives 'er all it's got, has to provide
a pressure surplus. If you are pump limited at idle then
you're hosed (heh) at the big end. Most GMs (and all
4th gen F-body donors) had non-manifold-referenced
fuel pressure and if you are putting vacuum (manifold
pressure) to the regulator then you need to fundamentally
change your IFR table, which as-stock models a variation
in fuel shot with manifold vacuum; a manifold referenced
regulator eliminates that trend (and it's not a bad thing,
can help consistency, but you need to model it right).
There are spreadsheets out there that can help you
with that.
The GM scheme has a blowoff regulator on the return
and the pump just gives 'er all it's got, has to provide
a pressure surplus. If you are pump limited at idle then
you're hosed (heh) at the big end. Most GMs (and all
4th gen F-body donors) had non-manifold-referenced
fuel pressure and if you are putting vacuum (manifold
pressure) to the regulator then you need to fundamentally
change your IFR table, which as-stock models a variation
in fuel shot with manifold vacuum; a manifold referenced
regulator eliminates that trend (and it's not a bad thing,
can help consistency, but you need to model it right).
There are spreadsheets out there that can help you
with that.
Thread Starter
Joined: Nov 2007
Posts: 413
Likes: 0
From: Albuquerque, New Mexico
The Fords of the '80s / '90s all were 3-bar (42-ish PSI).
The GM scheme has a blowoff regulator on the return
and the pump just gives 'er all it's got, has to provide
a pressure surplus. If you are pump limited at idle then
you're hosed (heh) at the big end. Most GMs (and all
4th gen F-body donors) had non-manifold-referenced
fuel pressure and if you are putting vacuum (manifold
pressure) to the regulator then you need to fundamentally
change your IFR table, which as-stock models a variation
in fuel shot with manifold vacuum; a manifold referenced
regulator eliminates that trend (and it's not a bad thing,
can help consistency, but you need to model it right).
There are spreadsheets out there that can help you
with that.
The GM scheme has a blowoff regulator on the return
and the pump just gives 'er all it's got, has to provide
a pressure surplus. If you are pump limited at idle then
you're hosed (heh) at the big end. Most GMs (and all
4th gen F-body donors) had non-manifold-referenced
fuel pressure and if you are putting vacuum (manifold
pressure) to the regulator then you need to fundamentally
change your IFR table, which as-stock models a variation
in fuel shot with manifold vacuum; a manifold referenced
regulator eliminates that trend (and it's not a bad thing,
can help consistency, but you need to model it right).
There are spreadsheets out there that can help you
with that.
Just for reference, the Ford uses a return system. I went with an older Silverado intake that has a built-in regulator (new) with a factory return set up.
Since the truck ran well at low to midrange for a year and a half, my plan to address the pump was to adapt a Walboro designed for the LS motor. While I don't believe it will solve my two cylinder dilemma, it should cure some fuel delivery issues that crept up from time to time. I ordered 2 new injectors to replace the two from the misfiring cylinders. They were cheap ($18 each). I'll do a compression check Friday and a leak down Saturday to isolate any mechanical issues.
Just for reference, the Ford uses a return system. I went with an older Silverado intake that has a built-in regulator (new) with a factory return set up.
Just for reference, the Ford uses a return system. I went with an older Silverado intake that has a built-in regulator (new) with a factory return set up.
Thread Starter
Joined: Nov 2007
Posts: 413
Likes: 0
From: Albuquerque, New Mexico
Yes....the PCM came from the harness builder with my build information. Curious....what PCM modifications would be needed if I went returnless?
