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Thread: 1993 Mustang Engine Compression Concerns

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    Senior Member NA5KAR's Avatar
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    1993 Mustang Engine Compression Concerns

    Thanks in advance for any thoughts. I bought a 93 mustang to use the engine / transmission for my Mk4 build. The Mustang was running. Drove and shifted ok. I would love to do a Blueprint engine, but trying to stay within a budget. The engine checkup, removal, surface rebuild, etc is a first time endeavor for me. The goal is to remove the fuel injection system and go with a carburetor. I want to simplify. A friend recommended a compression check before moving forward.

    20190616_122456.jpg20190616_122508.jpg

    Being new to compression testing, but after some research, it appears that the difference between the highest (140) and lowest (115) cylinder compression is 25 psi, which is 10 psi more than accepted parameters.

    So, here is the question ... Without doing an engine rebuild, are the compression numbers within usable tolerances? Beyond that, am I likely taking on more than I should for this part of the Mk4 build project? Please feel free to be harsh and direct. Thanks

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    On a budget.... Have you been driving the mustang? Do you care if you spread the cost out but get the car on the road and plan to pull the engine later to rebuild or replace.
    If its the difference of not eating or taking a second on the house, i would use it to keep the project moving forward.

    By the way, is the mk4 ready for the engine? What stage in the build are you at?
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    I just read you are just getting going on the build. Is the mustang still drivable? How many miles have you put on it since you bought it?

    If I had put some miles on it or still am, I would worry less than if I just bought it and the seller said it runs ok!
    Mk3.1 347 AFR 205cc Heads A9L EFI siemens deca 60lb injectors MSD 6AL ignition Vortech V-3 3 Link PS/PB

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    Senior Member weendoggy's Avatar
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    First thing you need to know is Ford is numbered 1-4 on the right bank and 5-8 on the left. Do a leak down test on the engine to get a better idea of condition on "why" you have discrepancies in compression.
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    Senior Member Norm B's Avatar
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    I have A few questions before commenting. Were all the spark plugs out for each cylinders compression check? Was the throttle held wide open during the test? Did you notice any oil smoke from the exhaust while you had it running? Do the spark plugs look the same or does #1 show signs of oil fouling? Any sign of coolant loss?

    There are many things that can cause a difference in compression readings besides cylinder wear. An improperly conducted test, bent valve, blown head gasket etc can all cause a variance in the reading.

    If you have the correct answers to my questions above then, I would try spraying a little fogging oil in #1 and turning the engine over a few times an repeating its compression test. A significant jump in compression would indicate wear. No jump means another issue.

    Good Luck

    Norm

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    Senior Member NA5KAR's Avatar
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    Just puttering: The Mustang was drivable but is not now. I took the exhaust manifolds off to get to the plugs and do the test. Was going to change them anyway. My Mk4 build is in the very early stages. Probably jumped the gun on buying the car, but I have it now. I've been tossing around the idea of selling it and buying a rebuilt ready to go engine.

    weendoggy: great observation. I only numbered the valve covers to correspond to the numbers I put on the spark plug wires so I would know where they go. I did not do a leak down test, but I absolutely will.

    Norm B: All spark plugs were out during test. Throttle was not held open during test. (remember - novice). Exhaust manifolds off the engine. No smoke while running. All Spark plugs looks pretty much the same. No coolant loss. I am aware of the 'small amount of oil' test. I did not do it, but I can make that happen.

    Thanks guys

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    Seasoned Citizen NAZ's Avatar
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    If you have no experience building or overhauling an engine then even thinking of that and you're already over your head. As an experienced engine builder with all the expensive precision tools required to do the job, I still can't build one for less than the cost of a crate engine. I'm building one now, but only because it's a custom build race engine that is significantly different than what the top tier sportsman race engine builders like Shafiroff and Schmidt offer.

    Bite the bullet and purchase a good crate engine -- it will be less frustration for a rookie and last as long as the car will.
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    Senior Member RickP's Avatar
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    I’m gonna second Naz. I had an 87 5.0. Had the lower end stroked and I built the rest. Being a first time go at any type of internal engine work, I quickly blew that sucker up only to have an experienced engine builder redo what I did wrong. In the end, I have the worlds most expensive 331 stroker. Bite the bullet when the time is right and purchase a crate motor. If you shove the current mill in the car now and have to address it one way or the other at a later time, you will most likely have to redo all the various systems which support whatever ends up in your build. A good solid trouble free engine will actually save you money in the long run. Don’t go the “would ah, could ah, should ah” route on your engine.

    RickP.

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    Senior Member NA5KAR's Avatar
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    NAZ and RickP, Thanks so much for the wise words. The Mustang goes on Craigslist later today. I'll do the right thing and will be happier for it. Just needed the push.

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    Senior Member RickP's Avatar
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    I think you will be much happier in the end. Engine builders who are connected to this community tailor engine packages specifically for our cars. Nothing better (and as Naz points out, less frustrating) than installing a turn key engine that you have dyno numbers for with a warranty attached.

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    Senior Member CraigS's Avatar
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    I disagree. Not that a crate isn't a good idea but engines w/ 25# differing compression have been driven many 10s of 1000s of miles. Ford 5.0s have a rep for lasting forever, especially the cylinder bores. At the very least I'd like you to redo the compression test as a technique double check. Two thoughts; 1- block the throttle fully open for the entire test. 2- be sure you have a strong battery/jumper car so you can, on each cylinder, keep cranking until the gauge stops increasing. (some say to crank for X number of seconds but that is guaranteed to give erroneous #s). W/ a tight budget in mind, you can clean that engine and paint it, replace front and rear crank seals, water pump, install a cheap chinese intake, and a QF carb, a new bone stock clutch, and have it ready to go for <$2000. Remember I included the clutch, intake and carb in that $2k and you would need to add them to a crate engine. So that $3500 (or whatever) crate engine still needs another $1000 added to it. Run your salvage engine for as long as you want in the FFR and sell it when you want to upgrade. It won't be worth a lot but, if you can take a prospective buyer for a ride to prove it's in good shape, (beats the heck out of buying a salvage engine that's been sitting for years) I bet $800 would be easy to get.
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    Senior Member NA5KAR's Avatar
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    Thanks CraigS. Words of wisdom. Been too busy to put an ad on Craigslist for the car yet. Perhaps I will ponder this for the rest of the week before making a final decision.

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    Mental note on the compression check. When you hold the throttle open, the computer will not fire the injectors, saving you from pumping gas into the cylinders during the test!

    Why not drive it for awhile during the initial build (gotta keep the fridge in the garage full). It will not take long to decide which way you want to go with it. I would be tempted to put the carb on it and drive it in the mustang some to make sure all is good. You keep the motor fresh while building the car and dont have to go through finding issues when you want to go cart it!!!
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    Senior Member NA5KAR's Avatar
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    Thanks Just puttering. Also great advice. My goal with the compression test was to learn if I need to rebuild the motor, or have it rebuilt. If the test was good, I could give the motor new life with seals, water pump, starter, alternator, distributor, intake manifold, carb, etc. If it needs a rebuild, I will be more inclined to sell the car and do a crate engine. I will try all of the recommendations given here to produce a more accurate compression test this weekend. FYI, the Mustang doesn't have a battery, so I use heavy duty jumper cables from my (2 battery) F250 to do the test. I can spin the engine until the needle on the gauge stops moving. I'll report back what I learn. I really appreciate your input.

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    Steve >> aka: GoDadGo
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    I had a Z/28 that I ran the crap out of from 1980 until it went swimming in 2005 during Katrina.

    I always used quality motor oil so I was able to rebuild it three times (Rings & Bearing Only) and never had to turn the crank or bore the block or change the pistons...That car did get bearings, oil pump, timing chain, cam and lifters for each rebuild and a head upgrade for the 2nd and 3rd overhauls but the rotating assembly had over 250,000-Plus miles on it when the car was lost.

    My feeling is if it isn't knocking and you don't have more than about .007-.009" taper on your cylinders you might be able to cut the ring ridge out, hone the holes, polish the crank and slap some rings and bearings in it...Just know if you hot tank a block you then need to replace the cam bearings too.

    My point is if this "Banker" can freshen up an engine, you likely can do it too.
    Last edited by GoDadGo; 06-19-2019 at 07:18 AM.

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    Senior Member CraigS's Avatar
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    Puttering has a great idea. You will buy a carb and manifold anyway so a small additional expense would be a pressure regulator to get the efi fuel pressure down to carb pressure. 40# or so down to 5-6#. It might be worth it depending on insurance, tags etc.
    Last edited by CraigS; 06-19-2019 at 06:34 AM.
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    David aka Ducky2009 Ducky2009's Avatar
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    Quote Originally Posted by NA5KAR View Post
    .......... My goal with the compression test was to learn if I need to rebuild the motor, or have it rebuilt.....
    Do another compression test, then squirt 1-2 ounces of oil into each cylinder (as you test each one) and see if the compression improves. This will tell you if the rings are not sealing well. As Steve aka GoDadGo suggested, rings and bearings aren't that difficult.

    PS: Add your location (city-state) and your build info into you signature. You'll find that people will offer to help if you're nearby. Most people also add a link (in their signature) to their own build.
    Last edited by Ducky2009; 06-19-2019 at 04:56 PM.
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    Senior Member NA5KAR's Avatar
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    GoDadGo, CraigS, Ducky2009, Love the input. Had family visit last weekend so didn't get to play with the car. Gonna try all the suggestions this weekend for sure. Thanks guys.

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