Power loss around 3000 RPM

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71coop

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Just about ready to throw  in the towel and take the car to a local mechanic who knows classic mustangs. No evidence of an exhaust restriction, poorly seated valves, or head gasket leak, based on both vacuum testing and one of those emissions detector kits you can get at Advance Auto (the ones that you stick in the radiator to detect combustion leaks into the coolant). The head gasket thought was a long shot anyway, my car doesn't overheat or miss - it just falls flat at cruise and 3K RPMs. 

As I thought back to when this first appeared, I realized the first time it happened was within a few weeks of installing my new (rebuilt) FMX transmission. I didn't drive much between the installation and about 2 weeks ago, so if it is the trans, that may be why I didn't see this issue sooner.  I have not considered this a transmission problem because it does not slip (My RPMs go down, not up). The TC is the same one I used with my previous FMX, so about 90% sure that's not the issue.The rebuild was done through a local rebuilder who has a lot of experience with classic car transmissions, but knowing the FMX isn't a popular choice these days, I wonder if that isn't a greater possibility of a mistake even by a good mechanic.

Any forum thoughts on whether a bad rebuild could result in some sort of drag on the motor at cruise?

The only other thought I have is a possible dropout in electrical power at 3K RPM, shutting off my coil and/or ignition module. battery and alternator look good at idle. 

Both of these ideas also feels like a long shot to me, but that's where I'm at... if  I don't find this by Wednesday, I'm taking it to a smarter person than me....

 
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Just about ready to throw  in the towel and take the car to a local mechanic who knows classic mustangs. No evidence of an exhaust restriction, poorly seated valves, or head gasket leak, based on both vacuum testing and one of those emissions detector kits you can get at Advance Auto (the ones that you stick in the radiator to detect combustion leaks into the coolant). The head gasket thought was a long shot anyway, my car doesn't overheat or miss - it just falls flat at cruise and 3K RPMs. 

As I thought back to when this first appeared, I realized the first time it happened was within a few weeks of installing my new (rebuilt) FMX transmission. I didn't drive much between the installation and about 2 weeks ago, so if it is the trans, that may be why I didn't see this issue sooner.  I have not considered this a transmission problem because it does not slip (My RPMs go down, not up). The TC is the same one I used with my previous FMX, so about 90% sure that's not the issue.The rebuild was done through a local rebuilder who has a lot of experience with classic car transmissions, but knowing the FMX isn't a popular choice these days, I wonder if that isn't a greater possibility of a mistake even by a good mechanic.

Any forum thoughts on whether a bad rebuild could result in some sort of drag on the motor at cruise?

The only other thought I have is a possible dropout in electrical power at 3K RPM, shutting off my coil and/or ignition module. battery and alternator look good at idle. 

Both of these ideas also feels like a long shot to me, but that's where I'm at... if  I don't find this by Wednesday, I'm taking it to a smarter person than me....
Well, I'm out of ideas too. I have no knowledge of FMX or any other auto trans for that matter. I keep wondering about the ignition module. Talk about a long shot, but what if you just happen to have two faulty modules that are limiting your rpm to around 3k. I know that sounds weird and really out left field, but I had a brand new never-been-out-the- box Pertronix II that was bad and a no start condition. Do you still have or can get a set of points and condenser and try that before you pay someone who may just do the same thing? If it is nothing to do with that, I'm flat out of ideas.

Somehow, I know we'll all be shaking our heads when we find out what the problem really is. Keep us posted.

 

71coop

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I may give the ignition one more shot, I've  had  things like that happen - I also agree once  this gets resolved, I'll probably kick myself for missing something small and obvious. 

 
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I may give the ignition one more shot, I've  had  things like that happen - I also agree once  this gets resolved, I'll probably kick myself for missing something small and obvious. 
Worth a try. Can't hurt anything. Are you sure the damper and pointer markers are correctly set to the crank (and cam) Your timing could be way off, but yet look correct to the timing light. I don't know, like you, I'm just throwing mud at the wall.  I think if it was your tranny, it would feel like the car is pulling back on you, like you had an anchor dragging behind.

It was running, now its not, why?

 

midlife

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I'm placing my bet on the carb: that the secondaries aren't opening due to some mechanical interference or lack of vacuum.  Yes, I know carbs were swapped, but the symptoms just reek of lack of fuel.  JMHO.

 

donkost

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I believe he said if he drops the trans into second gear he can go up to 4K rpms, so that still shoots down a number of the theories that are being revisited.

 
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I'm placing my bet on the carb: that the secondaries aren't opening due to some mechanical interference or lack of vacuum.  Yes, I know carbs were swapped, but the symptoms just reek of lack of fuel.  JMHO.
He can check if they're opening by placing a paper clip on the vacuum rod at the top and seeing if it has moved down after a drive. I believe he said the 4160 had vacuum secondary's. It's a weird issue for sure, but likely something simple and overlooked by us all.

 

mezell29

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I do have one other idea based on something new mentioned - You have the operational ram air. Have you tried running without that, maybe put on a open air filter? I saw you tested it, but perhaps the issue isn't fuel starvation but maybe there's some vacuum issue under load that is allowing all the flaps to close and its starving for air under power. Maybe a failed or disconnected vacuum canister. 

The FMX has nothing electronic in it, its  just a big hydraulic pump. Failure = not shifting or slipping. Anything that would limit rpms at this level would make horrific noise. I even tried a search on the web and couldn't find a single reference to a non-electronic transmission causing the conditions described, always the reverse due to slippage.

These cars are simple air pumps honestly. Air/Fuel in, combustion, Exhaust out. To oversimplify, The vacuum indicates the engine is in good condition as stable vacuum in a strong range. Your exhaust is free. This leaves 2 options - air/fuel or ignition. 

Something is failing under load or heat. Could be as simple as not having a resistor on a coil that requires one and the coil overheating. Bad plugs, wires, cap, timing, etc. Or both the current and replacement part have the same problem. Replacing with another one will produce the same results (Ask me how I know). I even considered a rev limiter (My MSD has one) but the OP has pertronics.

My money is on fuel. In the garage the car can keep enough fuel for revving above the 3k rpm mark. Under load at steady speed the engine consumes more fuel and its not happening. 

I had similar issues and had just replaced the entire fuel system - tank, lines and carb. Nearly $1000 so there shouldn't have been anything wrong.  I had put on a gauge and pressure was stable 6.5 lbs.  It would run for awhile but lose power after a few miles. I could run it in the garage for hours without issue. Out on the road, it would die again after a few miles. 

Turned out after 2 weeks of testing and trial and error to be vapor lock caused by a number of smaller problems. The fix required removing the clear fuel filter (They cause a low pressure condition)removing the mechanical pump, installing an electric pump at the tank to pressurize the lines, a heat shield, spacers and insulating lines in the engine bay. Granted I live in a much warmer climate but just drove almost 100 miles yesterday in 95+ heat with the AC on and haven't had an issue since I made those changes. 

Stick will air, fuel, spark and you will track it down!

 

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Did you try closing the spark plug gap to .035? This certainly is an odd one. You may want to check the coil wire to see if the connector made it through the 90 degree boot turn and is fully seating in the cap, yeah it is a long shot but at this point........  Chuck

 

71coop

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Stanglover - once I get time, will check the secondaries again. It's worth a try.

Mezell29 - I cannot disagree with anything you said, it has to be fuel, spark or air - I did check to make sure all the ram air hardware is correctly operating. I've fought issues with vapor lock and fuel boiling with the vette, and used a heat shield and return line to successfully deal with it. This behaves a little different, it is very consistently RPM-related when the car is under load. I want to lean towards fuel as well, but we'll see.

c9zx - I did check gap, it is .035, and just for shits and grins, threw a new coil, plugs, etc in - no change. I did not check the coli wire, though - will do that ASAP.

 

cv_72mustang

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Years and years like 35 years ago my 72 was running poorly over a certain rpm. Turned out to be bad wires, never forgot this since my friend and I were randomly poking around one late night  and saw plug wires  arcing in the dark. Very hard to see but for sure arcing. Course the plugs wires then are  nothing like today's.  Just a thought.

 

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Just trying to throw anything out there.  Does any ignition component you're running have a rev limiter capability that can be modified by the user?  Maybe it somehow became accidentally set to 3K rpms.  Of course that still doesn't jive with the fact that you can downshift to second gear and take it up to 4K rpms.  Is that still true?

 

71coop

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Donkost - I've never heard that the pertronix has a rev limiter. On the 4000 RPM in 2nd, it was true last time I tried,that was going into the weekend. AS I said before, I'm reluctant to push it too hard till I understand more,simply on the off chance it's not just air/spark/fuel. 

I tried a couple of the other suggestions last night with no joy. This afternoon, I am taking the car to a local mechanic who does a lot of the classic car work here in my hometown. I'm sure I missed something basic, but just don't  have the hours to spare right now, between work and other home projects I need to focus on. Time to let someone smarter than me take a crack at it.

Once he finds what  I missed, I'll post it here.

 

mezell29

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Just trying to throw anything out there.  Does any ignition component you're running have a rev limiter capability that can be modified by the user?  Maybe it somehow became accidentally set to 3K rpms.  Of course that still doesn't jive with the fact that you can downshift to second gear and take it up to 4K rpms.  Is that still true?
I thought about this too - I have a MSD with a rev limiter but the OP had a Pertronix. Unless that unit is bad somehow. I ran one in another car without problems but a failing unit could cause it. I am really curios about the solution to this one. I do a lot of really techie stuff and I have to solve a mystery!

 

73pony

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Which pertronix module are you running? I, II, or III? The III is junk and will cause this issue. 

 

71coop

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Which pertronix module are you running? I, II, or III? The III is junk and will cause this issue. 
it's not the III - I'm sure its the first-generation ignitor, it looks like a little black square module.

 

73pony

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Ok was just checking. 1st gen is good. 2nd gen is good as well but you have to either run via relay or bypass the resistor wire to get a full 12V to the coil. The 1st gen will run on the reduced voltage of the resistor wire without modification but the 2nd will not run properly and the 3rd is garbage, ask myself and several other members. I had a III and after about 800 miles it started doing as you describe. Basically it was not holding timing. Would idle fine but when reving under load the timing would fluctuate all over the place.  

 

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If it cuts out almost precisely at 3K rpms in a predicable manner, it almost seems like it has to be ignition-related.  That thought and 2 bucks (or whatever it is now) will get you a cup of coffee in any US diner.  lol  I used to troubleshoot this type of stuff in the early 80's when I tried to do all of my own work to save money I didn't have.  I had the plug wires arcing issue once that someone else described earlier.  I worked at a regional auto parts chain and had full use of a unused four-bay shop in back with a lift, plus unlimited parts access.  19 years old and no other responsibilities, sometimes I miss those days.  ha  Anyway, I'm thinking that if it was a fuel or carb issue and the car is being driven in different temperature ranges, this issue would not repeatedly occur at 3K rpms.  Keeps coming back to ignition in my mind.  It will be interesting to see what the pro mechanic comes up with.  Probably similar to your current situation, now I have plenty of money but no time.

 

71coop

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I took the  mustang to a local mustang guy - he called me back convinced it is fuel starvation. he took  the car  out, ran it until the power loss happened,then shut the car down. The fuel bowl on the carb was nearlyl empty - it would barely squirt gas when the throttle was manually rotated. This is  unlike at idle, just after starting, it is full and has a healthy squirt (he checked). 

Before going further, he called me and walked me through what he did - then, knowing I do a lot of my own work, he wanted to know if I wanted to take it from there. His opinion was carb needle and seat were sticking, or I had a (second) weak pump. He was leaning toward carb because of the pump swap, as was I, so I decided  to  take the car home and started in Saturday morning. 

I pulled the carb and cleaned it up - I did find a seemingly sticky needle. Put it together, and sonofabitch, same problem. Tried a third pump (not stock - an RPC 80 gph pump). No change.

I then tried a 3rd carb (I have a shelf full of 3 or 4 holley/QF carbs and a couple quadrajets). It actually showed improvement with a 670 cfm QF carb - but still had the power loss, just a bit later in the RPM range.

By now I'm ready to explode. Went  inside, watched a little TV with the wife,who was being exceptionally patient with me. I decide to goback to  square one. I had blown the lines out and was sure  they were clear,but there had to be a blockage somewhere.

Started a fresh inspection of my hard lines and eventually found this where the hard line passes through the wheel well:

wheelwell.jpg kinkedline.jpg

I missed it before, probably because the kink is back in the recess next to the spring,and was hard to see. All I can figure is that when I put the trans in, I must have pushed against the line and bent it.

The line isn't blocked, it will passs fluid and air, but that has to restrict flow - any feedback/thoughts on this being the cause of fuel starvation?

 

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