Confused about proper procedure to do compression test

billj

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#1
Make
Ford
Model
escape
Year
2005
Miles
113,000
Engine
2.3 liter 4cyl
I plan on doing a compression test on my 2005 Escape 4cyl. I am very confused as to the proper procedure.
I have read/watched videos that say to test the engine cold, another will say bring the engine up to operating temperature.
One will say to remove one spark plug at a time for testing, another will say remove all the plugs at the same time before testing
One site will say to have throttle wide open, another will use a remote starter under the hood and never mention the throttle position.

So what I need please is help from the experts out there. I know that I can trust your guidance.
 

billr

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#2
Hot or cold makes little difference, except your chances of getting burned. I generally do it cold.

All plugs out or just one out also doesn't make much difference if the battery/starter is good. However, unless you intend to check only one cylinder, you will be taking all the plugs out so wait to take compression until they are all out. Generally, that is the most common procedure, all plugs out.

The throttle should definitely be held wide-open; that will make considerable difference in the readings.

You may find various opinions about how many strokes to let it hit for each reading. I find that a good engine will hit max after 5-6 strokes.
 

grcauto

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#4
The best way to get the most out of compression tests is to look at the static with engine cold write down the readings of each puff, every time the cylinder is compressed, and note at least three full puffs. At this point you should be at over 90 percent of top compression. But allow for four or five and note it any cylinder is still climbing more. Do the same for engine warm. Then do a running compression test. and compare all results. You may get a better idea of the mechanical state of your engine using the vacuum method to test. Basically it's telling you what the pistons and rings, valves, exhaust, etc. and is much easier and faster to do. I posted an article I wrote on this just within a couple weeks. Should have an area where we can posts thee testing procedures. For the sake of looking all over for the article I will post it here.


Evaluating engine operation and pinpointing specific problems requires a comprehensive testing routine. Here's how to do it.
There's nothing more basic than the fact that an engine is just a big air pump. It draws in air by creating a low-pressure area in the intake manifold and cylinders, compresses the air, mixes in a little gasoline, lights a fire, generates heat and pressure and finally pumps out the spent exhaust. Our preoccupation today with things electronic sometimes makes us overlook old-fashioned mechanical symptoms of problems and the mechanical test equipment used to troubleshoot them. Vacuum gauges are often in this category, but the insight that a vacuum gauge can provide is as valuable today as it was 30, 40 or 50 years ago.
Remember that engine vacuum is just air pressure lower than atmospheric pressure. The starting point to evaluate engine vacuum is the intake manifold. When you connect a gauge to a tap on the intake, you're measuring manifold vacuum. Note that vacuum will vary in different areas of the engine, such as above or below the throttle valve and right at the intake and exhaust ports.
Vacuum drawn from an opening ahead of the throttle is called ported vacuum. Throttle opening affects ported vacuum opposite to the way it affects manifold vacuum. For example, at closed throttle, manifold vacuum is at its peak. But there is no significant vacuum at a port ahead of the throttle plate when the throttle is closed. Vacuum appears at such a port only when the throttle opens.
It's important to remember that manifold vacuum is used to power vehicle systems that need a steady supply of low-pressure air under all engine operating conditions. These systems include power brake boosters, a/c vacuum motors and some emissions controls.
Ported vacuum is used to control vehicle systems in relation to engine load. These include old-fashioned distributor vacuum advance diaphragms and carburetor assist devices. They also include many emissions control devices and transmission shift points. Under some engine load conditions, ported vacuum may equal manifold vacuum, but it can never exceed it.
Get Out the Gauge
Most vacuum gauges are graduated in inches of mercury (in.-Hg) and millimeters of mercury (mm-Hg). Some also show the modern metric scale of kilopascals (kPa). For comparison, 1 in.-Hg equals 25.4mm-Hg, or about 3.4 kPa. For this review, we'll stick to in.-Hg, or simply inches of vacuum.
Because engine vacuum is based on comparison with atmospheric pressure, it varies with altitude just as atmospheric (barometric) pressure does. The following table shows that as altitude increases, vacuum decreases about 1 inch for every 1000 feet above sea level.
Inches of Altitude Vacuum
Sea level-1000 ft. 18-22
1000-2000 ft. 17-21
2000-3000 ft. 16-20
3000-4000 ft. 15-19
4000-5000 ft. 14-18
5000-6000 ft. 13-17
Normal manifold vacuum at idle for an engine in good condition is about 18 to 22 in.-Hg. Manufacturers used to publish vacuum specs in service manuals, but this isn't as common as it was years ago. Still, the physics of internal combustion haven't changed in a hundred years, so the guidelines given here are a good starting point for vacuum gauge troubleshooting. Your best analysis based on vacuum readings will come from your own experience, however. As you use a vacuum gauge on different engines, you'll learn what's typical for one model compared to another. Some engines have reputations as low-vacuum motors; others are unusually higher than average. Experience is your best teacher.
Cranking Vacuum & Speed Tests
You can get a quick basic appraisal of engine condition by connecting a vacuum gauge to the manifold and a tachometer to the ignition to check vacuum and rpm at cranking speed. Warm up the engine first, then shut it down and connect your test equipment. Close the throttle and disable the ignition, or use a remote starter so the engine won't start. Crank the engine for 10 to 15 seconds and observe the vacuum and tach readings.
Note that different engines produce different cranking vacuum readings. Some carmakers publish specifications; others don't. Again, experience will be your best guide. What you're looking for, most importantly, is steady vacuum and cranking speed.
If the cranking speed is steady (about 200 rpm) and vacuum also is steady (around 5 inches), the engine most likely is in good mechanical condition. If rpm and vacuum are uneven, the cylinders aren't pumping equally. The engine probably has leakage past the valves, rings or head gasket. If the vacuum reading is pretty steady but cranking speed is not, you're probably looking at a damaged flywheel ring gear or starter. If the cranking speed is normal or high but vacuum is low and slightly uneven, the engine probably has low compression or retarded valve timing. A jumped timing chain or belt is a common cause here.
The cranking vacuum test also can provide a quick test for PCV restrictions. Perform the test and note the average vacuum reading. Then pinch the hose to the PCV valve closed with your pliers and repeat the test. If the PCV system is clear, vacuum should increase. If it doesn't, check the PCV system closer for restrictions.
What Idle Tests Can Reveal
You can zero in on several basic mechanical problems by taking a quick look at manifold vacuum. Warm the engine to normal temperature-get it really warm-and connect your vacuum gauge. Make sure you connect to a manifold vacuum tap and not to ported vacuum. Connecting a tachometer also is a good idea.
Just to be sure that the evaporative emissions system doesn't interfere with vacuum testing, disconnect and plug the canister purge hose and its manifold port. If you're testing an OBD II car, check for evap-related DTCs when you finish testing to be sure none set.
Run the engine at idle, low cruise (1800 to 2200 rpm) and high cruise (2500 to 3000 rpm). Note the vacuum readings, and any fluctuations, at each speed. Next, hold engine speed steady at about 2500 rpm for 15 seconds and read the gauge. Now release the throttle and watch the gauge as the speed drops. The vacuum reading should jump as the throttle closes, then drop back to its normal idle reading. If vacuum doesn't increase at least a couple of inches when you release the throttle, you may be looking at worn rings, cylinders or valves.
Idle vacuum for most engines is about 18 to 22 in.-Hg, but some may produce only 15 to 17 inches at idle. (Remember what we said about experience.) If vacuum is steady and within these ranges, the engine and fuel and ignition systems are operating normally.
If vacuum is steady at idle but lower than normal, the ignition or valve timing may be retarded. Low compression, an intake leak or tight valves also can cause low vacuum at idle.
If the vacuum reading fluctuates within the normal range-the gauge needle bounces around a lot-uneven compression (broken rings or leaking valves or head gasket in one or two cylinders) is a likely culprit. An uneven air/fuel mix, erratic ignition timing, a misfire, misadjusted valves or a manifold leak near one or two cylinders also are possible causes.
If vacuum drops intermittently at idle, one or more valves may be sticking open or dragging. Higher-than-normal vacuum at idle is a common clue to overly advanced ignition timing, while low vacuum can indicate retarded timing.
Low vacuum also can be an immediate clue to a plugged exhaust. To check further, run the engine at about 2500 rpm for about 15 seconds. If vacuum drops during this period and does not increase when you close the throttle, you're almost certainly looking at a restricted exhaust.
Vacuum Fluctuations & Power Balance
Several of the guidelines in this article have distinguished between steady vacuum gauge readings and fluctuating readings, where the gauge needle bounces up and down erratically. This may seem secondary-almost inconsequential-but it's an important distinction. A steady but abnormal vacuum reading indicates a problem common to all cylinders. Things like incorrect ignition timing or an old, tired, high-mileage engine affect vacuum equally for all cylinders. A bouncing needle, however, usually indicates that the problem is localized to one or just a few cylinders. Here's where power balance testing enters the picture.
Compression testing on many late-model engines is flatly impractical from a labor standpoint for a quick engine evaluation. That's especially true on some of the weird vans for which removing and reinstalling spark plugs is a two-hour job. It's relatively quick and easy, however, to connect a vacuum gauge to the manifold and your engine analyzer to the ignition system.
If your initial vacuum tests produce gauge fluctuations, you have a definite indication that the problem is limited to just one or a few cylinders. In these instances, a power balance test can help you pinpoint those cylinders and the condition they're in. Does the engine need a valve job (fluctuating vacuum) or a complete engine exchange due to universally worn rings and cylinders (steadily low vacuum)? Combine modern power balance testing with traditional vacuum analysis and you'll have the answer.
 

nickb2

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#6
The throttle should definitely be held wide-open; that will make considerable difference in the readings.
To add to this comment, WOT is also great for fuel injected cars as most of them will kill fuel injectors by going to into flood mode. Meaning, when you think your engine is flooded, simply press gas pedal WOT, may help un flood the engine, if that can be said in english. My brain could not find a better term.

So by doing this, it is not necessary to disable the fuel circuit as going WOT takes care of that, so while doing compression test, you will not be injecting fuel into the cylinders, which also affects the readings.

Now this will probably not apply to those who don't have a labscope, but taking relative compression readings this way is also a great diagnostic tool.

I am adding this as an addition also to the great info GRcauto provided. So it is for informational purposes only.

I love taking relative compression readings with a scope as the hook up is just too easy when trying to diagnose a misfire. Simply hook the low amp probe to the positive bat starter lead. No need to remove spark plugs, or hook up a comp gauge etc. However, a relative compression test does not give you actual PSI. You can see on the scope the % difference. To get psi readings with a scope will require actually removing a spark plug and getting a physical reading as traditional, and then doing a bit of algebra to do a comparison. I will post a video I found on you tube to better explain that. Some engines have one bank hidden/not accessible, but one bank will be very easy to get to.

So lets say a car comes in with a check engine light on. It say P0302. We know that means cyl #2 is missing. Also, lets say to get to that plug, intake removal is needed to get to the plug, this is where a relative compression test shines. If you get consistent readings on the scope for each "bump"/cyl, we now know it is NOT a compression problem and we can now focus on faulty injector, spark etc.

So, here is that video. His scope is not very high end, but does the trick. Another great resource is a guy called Scanner Danner. I hear he comes up north sometimes to give classes, have yet to find one I can attend, but I do love his videos.

But I like Matts videos also, he is great at putting complicated stuff into layman's terms. As seen in the following video, it is a proven method of getting real psi data from scope amp data once one cyl is read in the traditional sense. Again, very usefull for engines with no access or limited access to plugs.

Take for example a grand caravan where the rear bank is horrendous to get to, but the front bank is in your face. Another example, mazda has engines where intake removal is neccesary just to get to the coils, much less remove a spark plug. Of note, not only mazda. More and more engines are designed this way today.

 
Last edited:

nickb2

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#7
Another prime example where this shone for me was on a ford taurus awd pursuit car. Engine. 3.5l. The front bank is very easy to access. Rear is impossible to get to as the upper intake is in the way.

It came in with a misfire for cyl 3. I did a relative compression test with my autel maxysys and the 4ch scope that comes with it as an option.

I instantly saw that it was a compression issue on cyl #3. Which is the rear bank when facing engine. No way to get to it to do a traditional reading without removing the upper intake. So by removing a front bank spark plug, took gauge reading there and did the math. Proved the engine was in very good health except for #3.

Ended up being a rounded cam lobe which I still have a picture of in my SD card on my autel.

So, it took me 2 minutes to whip out the scope. 2 other minutes to turn it on and hook low amp to B+ starter wire. 2 other minutes to gather data. 5 min to remove coil and plug and hook my comp gauge and get an actual PSI reading on cyl #4(front bank). Another 5mins to do the math. That's 16mins real time diagnosis. A real time saver when the shop is full. Book time for upper intake removal is 1.1hrs. So I shave more than an hour shop time and billable time to come to the same conclusion using the relative compression method and a bit of math. Not bad if you ask me.

Mind you, you do need access to a scope. Preferably an automotive one. Such as the preferred PICO scope or a snap versus etc, or my favorite all in one autel kit.

We went on the safe side and put in a new head. It is a cop car after all. If it would have been my personal car, I would have tried a used cam. But the local law enforcement usually has a good budget. I don't. ;)
 
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nickb2

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#8
Again, I must use a disclaimer here because the thread is called "Confused about proper procedure to do compression test" and probably could have been answered in a section that GRCauto aptly mentions that there is no section for tutorials.

I did try some tips and tricks for beginners back in the day. That was put into the open discussion I think. Many hits on that. But often missed by the general public and could have gotten more traffic if a section was included such as "tutorials, tips and tricks" for example.
 

grcauto

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#10
Do you all remember: dwell meter, timing light, points, distributers, spark plug wires. These vehicles I would tune by book and then retune by ear, ……...memories bad or good is the question
I found there were better ways to adjust the points on GM Chevy V engines than using a dwell meter. Turn in the points until the engine just starts to skip and back off a half turn and power brake the AT and adjust to highest stall point. For MT we would do snap throttle until you could get the fastest response without lag. They ran better and had better times than the same if dwell were at specs. Had a good way to adjust some of the carbs using similar tactics.
 

©hester

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#11
I plan on doing a compression test on my 2005 Escape 4cyl. I am very confused as to the proper procedure.
I have read/watched videos that say to test the engine cold, another will say bring the engine up to operating temperature.
One will say to remove one spark plug at a time for testing, another will say remove all the plugs at the same time before testing
One site will say to have throttle wide open, another will use a remote starter under the hood and never mention the throttle position.

So what I need please is help from the experts out there. I know that I can trust your guidance.
You may want to pull the fuse to the EMC before cranking.
 

nickb2

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#12
Do you all remember: dwell meter, timing light, points, distributers, spark plug wires. These vehicles I would tune by book and then retune by ear, ……...memories bad or good is the question
Yes I do.

In fact. I am 43 yrs old and in principle should not know much about dwell and tuning cars by ear.


2018-1992=26. 43-26=17 Which takes me to when I started and got apprentice cards and all that jive. I was working par-time from 15-17 in a sears garage back in hamilton ontario.

!994, That was the year I came out of auto trades school up in quebec city where I was born. Still alot of carbs on the road but feed back carbs were the big thing back then and all the old guy's were putting locks in the tool chest when that came out. They had had enough with EPA rules from the 70's.


We had tons of gas guzzlers up here back then.
 

nickb2

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#13
I still, to this day still use my first and last timing light kit from equus I ever bought (did NOT WRITE ever used). Stainless finish. Yellow neon box set. Came with tach, dwell, NON-adjustable timing light. Bot space was reserved for vac gauge, remote starter button, compression tester and adapters.

That was a REAL USA CANADA MADE shyte.

Still use it very periodically. The young bucks, when they see an older car come in. They just gravitate towards me and my neon yellow box.