Compression test 94 Dakota 3.9 Magnum

[Chris]

I posted about issues related to this previously. I ended up paying for a motor check to find the source of my issue they told me it was the tranny and referred me to a guy. That is not working out and the tranny guy is insisting it is the motor.
Today I have done a compression test, dry then wet. The info I could find on it said the OEM compression should be 100psi. That sounds Way low to me and these results indicate it may be wrong unless I do not understand something. I did not get the same amount of oil in each cylinder and I may have used too much so??? This is the first time I did it. Number 1 and 2 plugs had oil on the threads the tips of the plugs looked normal for almost 18+ months old.
Dry Test on left Wet Test on right in order down the sides.
Cyl 1- 135psi 172psi
Cyl 3- 132psi 179psi
Cyl 5- 140psi 185psi
Cyl 2- 105psi 142psi
Cyl 4- 130psi 185psi
Cyl 6- 122psi 165psi

The best I can imagine is the passenger side head gasket is leaking. Wost probably rings or a rebuild. Anyone?

Topic

[Mike]

Well, it’s 21 years old, after all. No surprise the compression readings seem a bit low, but they are pretty even with each other, with the exception of cylinder # 2. I’d say it’s showing more or less normal wear for its age, typical bore and ring wear.

[Chris]

I took it to a mechanic. He looked at those readings and told me how badly I was screwed. He had just rebuilt one for a guy 2 weeks ago $4K.
Then I asked him to listen to it. He then said “there is no way this motor has those readings.” He figured I either did it wrong or it was the Harbor Freight gauge I used. He then referred me to a guy who also said the motor was good. Truth be told they only listened to it.
Then I called him to ask if he would do a compression test and he told me it would be a waste of money “I’ll take your money but I think it is a waste”. He said to change the oil and put in one quart of Risolone or Mystery Oil for one of the quarts. He also suggested I look under the valve covers. Then he gave me the name of a tranny guy “Chrysler expert” The tranny guy hooked up a computer and we took a drive and it seems that on the flats at least it is acting normally. He explained how I could do adjustments of the TV cable. We talked about the one side being weaker but he had no ideas of how to determine what was the issue. All that came up was leak down test. He did mention that while driving it felt like a cylinder was not firing right. I’m hoping it has something to do with the injector synchronization. I’ll know Monday afternoon I finally found a guy that will do it..
He also thought that maybe the alt could be going off and sending a bad signal through the system which would cause all the random stuff. I told him about how the battery was swollen when I bought it and how the charger says it is overcharging and he really lit up. He told me to buy a good digital something electronic reader and find out what was running through my system saying it should not be more than 14 volts when running.
I got the oil changed (it was low@ 900 miles short of 3K since last change. The tech said 1qt but I checked earlier and it was halfway up the safe zone with the engine warm. It was hot for several hours when he saw it.) & they told me my rad cap was bad (That’s weird since it was good when I did the timing chain & plenum) and I replaced a turn signal forgetting to get the digital reader. One thing I did during the compression test was to unhook the battery and turn the key for 30 seconds to make it brain dead. Since then it seems a little better. But then it has improved each time I have done something for a little while.
So it may not be such dire news
I will get to trying a leak down or having someone do a leak down. I’d like to know if it is the head or the rings leaking. I suspect the rings may be sticking since the previous owner said it sat a lot for about 10 years.

[John]

An easy way to check for a significant issue with a cylinder is to just make sure both valves are closed then pressurize the cylinder. I just remove the valve from my compression gauge air hose, screw it into the cylinder, and hook up an air compressor the other end. If you hear air leaking through the oil fill hole or the dipstick you know your rings or piston are shot. Air leaking out the intake = likely bad intake valve, air leaking out the exhaust pipe = likely bad exhaust valve, etc.

[Chris]

Thank You. I only have a small compressor it wouldn’t hold enough pressure and I do not have the gauges necessary. I am going to try to find a shop that will do a proper compression test and then if its similar results ask if they will do the leak down.
Does anyone know if that info I found about the compression for this motor being 100psi is correct? Or am I reading this page wrong? http://dodgeram.org/tech/gas/specs/3_9v6.htm

[Andrew Phillips]

100 psi is the service limit. That means it shouldn’t be below 100psi. As with most engines, a max compression is rarely specified, and nominal compression figures are based on accumulated evidence. The rule-of-thumb way to calculate an approximate “max” compression pressure is to take the static compression ratio of the engine and multiply it by 20 (this is only a theoretical max, there really is no such thing). For your engine, the compression ratio is 9.1. Multiplying that by 20 yields 182. A brand new engine will have a bit higher compression than a worn engine, so a “respectable” compression would be more like 15 times the compression ratio to account for wear. 9.1 times 15 yields 136.5. Therefore, any pressure reading between the service limit and the theoretical max would be acceptable, with the ‘expected’ pressure to be around the 15x amount, as long as all cylinders are exhibiting the same pressure within 10%.

The key things that mechanics look for in a compression test are: pressures are above service limit, and consistency across all cylinders. In your case, the consistency is the stand-out. In a proper functioning engine there should usually not be more than a 10% difference between the highest and lowest reading across all cylinders. One thing to realize, all engines leak compression, even brand new ones. The cylinders are not completely sealed. This is due to the gap in the ends of the piston rings. This is also why engines have “blow-by” observed in the crankcase. As the rings wear, this gap increases, reducing compression. The main idea is that the rings should wear evenly, so the compression loss should also be even. That is why consistency is one of the key indications. The service limit comes from the expected amount of wear on the rings before the engine is no longer capable of building sufficient compression for reliable firing.

Most of your cylinders, dry tested, are running really close to the expected 15x number (due to the ring gap). Wet testing, which uses the oil to seal the rings, gives you a number close to the 20x number ‘theoretical’ max. Because of the ring gaps, even brand new engines will see a higher wet test than dry. Again, it is the consistency that is looked for, and the wet test is used primarily to see whether the compression is leaking through the rings or valves. If the compression is leaking through the valves, the wet numbers will not significantly increase because the oil isn’t sealing the valves, only the rings.

[Chris]

Thank You.
I knew a lot of that from experience an dErics videops.
I am hoping I can slow walk the shop into doing the leak down test because these motors apparently had head cracking and gasket issues. I’m hoping against reason that the problem turns out to be the valves or head. If nothing else it running so smoothly and being fairly well maintained until it got parked for most of the 10 years before I bought it might mean that I can just get new rings and maybe a valve job. Fingers crossed.

In case I have to rebuild which is better/cheaper, rebuild this one or just get a rebuild motor to install?

[Andrew Phillips]

Depending on the amount of damage, it may be better to drop in a rebuilt motor. Also, depending on the condition of the rest of the vehicle, you might even consider a used (junkyard) engine. Most of those come with some type of time-based or limited warranty. If your damage is limited to the top end, head work may be all that is required, which would cost significantly less, in most cases.

[Chris]

Thanks. I’m waiting for a call to let me know if I can get the compression/leakdown done today. I have an appointment with an electrical diagnostics guy who is going to make sure the electrical system is functioning properly.
I think I mentioned earlier that the tranny guy thought the Alt could be bad and said I should try driving with the Pos wire disconnected. I did and it improved how it shifted and had more response at the throttle. It also felt weak going up a hill without trying to accelerate I just held it steady. The Kickdown hesitated but it was still better.
Edit- I spoke to a mechanic and he felt he should just do the Leakdown test to pinpoint the exact type of problem and I will go in next week.

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