Question about 1965 Ford Mustang

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High compression in 1 cylinder of a 170 ci. 40 psi high.

Did a compression test and #4 cylinder was 40 psi higher than the other 5. They averaged 160 psi.

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  • Anonymous Mar 14, 2014

    compression test, how many psi should be in each cylinder?

  • Anonymous Mar 16, 2014

    Is compression supposed to be higher than 70 psi.



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Per the Ford manual, the acceptable range is 155-195 PSI. It sounds to me like you don't have a compression issue with this engine (without knowing the exact readings for each cylinder). Normally one cylinder being higher than the others, but still within spec, would not concern me a lot. Had one been much lower, then I think that might be cause for concern. There could be many reasons for the one hole being higher. If the engine hasn't been rebuilt in a long time, there could be carbon deposits in the compression chamber. There are many chemical fixes for this availble at your local parts store. If the engine is running fine with no pinging or detonation, I'd just continue to enjoy your classic ride (I own a 1971 Challenger converitble with a 440-6 pack, so I understand the joy of a classic car!).

Good luck and please let me know if this helps!


Posted on Oct 10, 2008


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I have a 88 b2200. It shudder, jumps in all gears, has power loss and gains and poor milage. I've rebuilt carb, changed plugs and wires, disributer and pvc valve. Any ideas. Thanks

I would do a compression test of the engine. Spend about $40 for the gauge tester and do it yourself. Engine should be at operating temperature, remove all spark plugs, disable ignition by grounding the high tension cable from coil to distributor, screw in the gauge to one cylinder, hold the gas pedal to the floor-or hold the throttle at wide open, then crank engine for about 5 seconds. You will have a compression reading that should be somewhere between 100 psi and 200 psi. Anything below 100psi means not enough compression in the cylinder to produce power when running. Do each cylinder the same-crank the same amount of time for each cylinder( with wide open throttle). All compression readings should be fairly close-lowest reading must be within 75% of highest reading cylinder for a good, healthy engine.

If compression is low in any cylinder, it probably won't even fire the air/fuel mixture. Look for valve problems, valves not sealing (burnt valves cause this), or piston rings worn too bad, head gasket failure can cause low compression. Example, compression tested a Nissan, and readings were all 155-160 psi in all cylinders, so compression was excellent, engine was internally good. If you have good compression in each cylinder, look elsewhere like a plugged catalytic converter-won't let the engine breathe, or a problem with the ignition system-test for a snapping, blue spark when cranking the engine, on one of the spark plug wires. Others: check EGR valve for being stuck open, good luck, then.

Apr 06, 2015 | Cars & Trucks

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I have a 88 Engine 351 ford with 70.000 miles on it when I check each cylinder what should my compression psi be to tell me if its worth puting money into it

petrol compression pressure should be around 160psi average ( low around 140 and high around 180
the difference between any cylinders should not be more that 10% so if the first and last readings are 160 and 145 that would be average and if the adjacent cylinders read low then that would indicate a gasket problem
The result should be that they are all very close in readings.

Feb 08, 2015 | Ford Cars & Trucks

1 Answer

High co reading for emission

what rpm or speed, both? which Civic engine????? of the 5
and what country????? we can not guess this... ever.
and no one ever says.. here. 99% true.
why is not easy. 50ways there are.
it's running rich. (we have one good input. this)

but here are the reasons, (not the 50 causes) all the below are tests.
it's misfiring. is rich misfire.
causes: many I'll assume the cars 12v power is 10 to 145v at all times. 13.3v running minimum.
1: low compression on one or more cylinders. 150psi w.o.t is min.
2: bad spark or not timed right. to any cylinders. or all. tests.
3: bad fueling. above must be good (tools used and tests)

now if compression is good. and the EGR main down leak
if spark is good, what causes high HC,
simple misfire.
what causes misfire.
  1. bad spark plugs (ever tune it up? now be the time, huh?)
  2. bad injectors, leaking or clogged. clean em.
  3. fuel pressure too high, 28 is min, max (see end)
  4. you find one injector , NOID lamp test stuck ON.
  5. o2 sensors stuck lean (exhaust leaks)
  6. Fuel pump regulator stuck closed, causing #3
there are 5 engines, (why not state yours.??????0
1.5 L to 3.0L
the fuel pressure is different on all .
40-47 PSI. 1.7L and 2.3L
47-54 PSI on 2.0L and 2.2L
41-48 PSI on 3.0L
and then the exceptions
1-- Except SULEV: 48-55 psi SULEV: 47-54 2 Except SULEV: 47-55 psi SULEV: 48-55 psi
if you read the FSM,, it shows tests at the rail.
fuel pressure.
fuel regulation test.
shut test
injector leak down test.
and the injector balance test.
we do them all, or fail;.
all this is FSM.

do you want us to guess or test?
the latter avoids the 50 guesses. 49 being wrong.$$$$$$$$$$$$$

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1 Answer

Popping on acceleration

It could be a problem in the valve train. For a car that old, before putting new parts in, you could buy a compression tester for about $30, and do your own compression test. It should tell you if the engine is good enough internally to warrant proceeding. If compression is low on any cylinders, that problem must be addressed first. If a valve is not sealing and causing the popping, that cylinder will show low compression.
For the test, the engine should be at normal operating temperature. Then pull all the plugs out, connect tester to #1 spark plug hole. Have a helper hold the throttle wide open while cranking the engine over about 5 seconds. Watch the gauge-the first revolution or two will cause the gauge to jump high at first, and successive revolutions during the 5 second span will cause the needle on gauge to climb a little each time. Record or note the compression on #1, then do each cylinder. Try to do the same amount of cranking on each cylinder, and keep the throttle wide open. What you want are compression readings on each cylinder that is well above 100 psi, and all should be close. The lowest reading should be within 75-85% of the highest reading cylinder. Example: say your highest reading is 135 psi. 85% of 135 is 115-so your lowest reading should be at least 115 psi. Some general guidelines: any cylinder with compression below 100 psi is definitely a problem-that cylinder will not have hardly any power output, even if it is firing. Worn piston rings or poor valve sealing is causing compression loss. A newer engine should have compression readings above 150 psi. As the engine wears, compression will drop. In general, for high mileage engines, you would want to see compression in the range of 125-150 psi, the higher the better, but remember, it should be balanced-the lowest should be at least 75% of the highest reading cylinder.
Post back if you have questions-good luck.

Jan 26, 2014 | 1983 Oldsmobile Firenza

2 Answers

I have a 99 sienna and the cylinders have low pressure what can i do?

If cylinder compression is too low, the car may not even start because of it. Have a shop do a compression test or a leak-down test to check the internal health of your engine. A good engine should have pressures of 150-200 psi in all cylinders and be close to each other. A worn or high mileage engine may have pressure as low as, say 125 psi, but if still balanced (close readings), would still run decent. Compression below that, especially below 100 psi, is serious trouble. ignition spark refuses to ignite the air/fuel mixture and you get a misfire-a non-working cylinder. Low compression can be caused by worn piiston rings or poor valve sealing. Low compression in 1 or 2 cylinders can be a valve problem, burned valves or valves not seating properly.
A leak-down test will check each cylinder and if there is a problem, the test will find the cause of the problem.

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1 Answer

What is the factory specified max. compression, new motor, GM 4.3 ltr, V6

Interpreting the Results
of the Engine Compression Test.The idea behind the Engine Compression Test (in the case of a Misfire Condition or Rough Idle Condition) is to find out if any one engine cylinder is not contributing 100% of its power to the overall engine output. If only one cylinder has lower than normal compression, your GM 4.3L (or 5.0L, 5.7L) equipped vehicle will run rough or cause a Misfire Code to set and turn on the Check Engine Light.
So then, to wrap things up, the individual cylinder compression readings of each engine cylinder can not vary more than 15%... and this is how you can find out:
  1. 1Grab a calculator and multiply the highest compression reading that you recorded by .15.

    So, let's say that cylinder #4 gave you the highest reading of 170 PSI. Well 170 X .15 gives you 26 (25.5 rounded off).
  2. 2Now, the next step is to subtract 26 from 170... which gives us 144 PSI.
  3. 3So then, 144 PSI is the lowest possible compression reading that any one of the rest of the engine cylinders can have. Any compression reading below this.. and that engine cylinder will misfire..

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Computer diagnosed o2 sensor, but problem is worse since changing, engine reves for about 1/4 mile before shifting

Check the transmission. The O2 was a problem but wasn't the root cause of teh problem. Also check the compression on all cylinders to make sure it is within 10 to 15 lbs of compression on each cylinder. ie If you have 135 psi on cylinder 1 all the cylinders should be between 125 and 150 psi. Extremely high or low psi on anyone cylinder when compared to the rest will cause this problem. You have described a situation where the motor is only running on 3 cylinders in a 4 cylinder motor or 4 to 5 cylinders in a 6 cylinder motor. The higher RPM is needed to get the same speed and result as a lower rpm in a motor using all cylinders on the motor.

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What is the cylinder compression psi fpr the 2.0 liter engine

usual on 4 cylinder around 175psi.225psi is very high for your car,150psi seems ok,you may have two problems.A leaking head gasket and high compression due to carbon build up on the pistons.Check your Hyundai Dealer to see is this problem is covered under their 100,000 mile warranty,it should be.IF not then
the head needs to come off and had gasket replaced along with clean the top of pistons since they are right there,sounds like and easy fix.also check condition of radiator,and I would rec. replacing thermostat since some Hyundai models had problems with their thermostats

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4.3 Liter V-6 Idles rough when in gear, runs fine on highway.

Can you check fuel trims at idle and under load?
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If fuel trim is high at high speed and loads, look for fuel volumn problem or restriction.
If fuel trim is high at all speeds and loads look for pcm input fault, maybe a maf sensor problem?
If fuel trim is negative low speed and load, look for fuel pressure regulator or injector leak. If fuel trim is negative at all speeds and loads check ect and iac.

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