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2G Odd Compression Result

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spyderdrifter

10+ Year Contributor
5,267
711
Jul 11, 2009
Somewhere in, Colorado
I'm posting this since currently, the search options seem to be broken and I can't search anything. Anyway, I ran a dry compression test just to get a basis of my engine's health. The results were as follows:

Cyl 1: 147 psi
Cyl 2: 143 psi
Cyl 3: 100 psi
Cyl 4: 150 psi

From there, you can all see the problem. But there's more. I didn't do a wet test, because I don't think cyl 3's reading is accurate. The other 3 cylinders are well within spec of one another, and they were higher than I had assumed they'd be, so I'm happy with those. However, while cranking the car over, cyl 3 would make a popping sound and wouldn't go above 100 psi. Every time yielded the same result with no variations. The only thing is I don't remember if the pop came before or after hitting 100 psi. I failed to note that. The car's power has been pretty good, but still at times it does kind of feel like it falls short. Not always, but on rare occasion. I don't know where this pop noise came from, and have no idea what could be causing it. The only idea I've been able to come up with, is if the pop happens right when it hits 100 psi, could one of the injector seals be popping and then re sealing? However, there's no leaking once it hits 100, and just simply won't go higher either. I'm a little lost on this one. The engine was warm as I had just returned fro ma test drive, and the only thing I forgot to do was open the throttle plate. Injectors were all unplugged and the plug wires were all pulled from the coils.
 
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As much as that may seem like the obvious thing to do, I've never had any compression results differ between throttle open or closed. More so with 3 of the 4 having decent results and one is way off, and the test was performed the same across all, I doubt the throttle plate is the issue here.
 
Okay well the the reason why I say redo the compression test correctly is so that we can get a proper Baseline. If this is a 8.5 - 1 CR 2G engine stock Factory compression is 178. Service limit is 133. If number three was that far off the car typically would miss and shutter and run rough. The injector seal is not going to affect your compression test results whatsoever. It will affect drivability because of a massive boost leak. What I would do after doing the compression test correctly at wide open throttle and getting the same results, is a leak down test to find out where the compression is escaping from in #3. When doing a leak down test make certain that you are on the compression stroke of the cylinder at top dead center and the valves are completely closed for that cylinder. Verify mechanical timing before doing leak down test.
 
I don't know if it would work or not. But maybe try some soapy water around the injector seals while the engine is cold and start it. See if there's any bubbles coming out.

Not a bad idea. I got a spare set of new ones I want to install anyway.

Okay well the the reason why I say redo the compression test correctly is so that we can get a proper Baseline. If this is a 8.5 - 1 CR 2G engine stock Factory compression is 178. Service limit is 133. If number three was that far off the car typically would miss and shutter and run rough. The injector seal is not going to affect your compression test results whatsoever. It will affect drivability because of a massive boost leak. What I would do after doing the compression test correctly at wide open throttle and getting the same results, is a leak down test to find out where the compression is escaping from in #3. When doing a leak down test make certain that you are on the compression stroke of the cylinder at top dead center and the valves are completely closed for that cylinder. Verify mechanical timing before doing leak down test.

See I thought the same thing with the stuttering and miss fires, but it drives really well and idles perfect. I have no issue doing a leak down, but I don't have an air compressor to do it with. I got the tool I made years ago still, but no way of doing the test. Definitely would have done it by now if I did.

Pull your valve cover and check cam lobes and rockers on that cylinder also.

And what would I be looking for if I did? Just normal wear or pitting?
 
And what would I be looking for if I did? Just normal wear or pitting?[/QUOTE]

I would say just anything out of the ordinary and it may not be obvious so just look at every valve, see if it looks like they are all the same height and that rockers are all in place and all look the same.
 
I'm posting this since currently, the search options seem to be broken and I can't search anything. Anyway, I ran a dry compression test just to get a basis of my engine's health. The results were as follows:

Cyl 1: 147 psi
Cyl 2: 143 psi
Cyl 3: 100 psi
Cyl 4: 150 psi

From there, you can all see the problem. But there's more. I didn't do a wet test, because I don't think cyl 3's reading is accurate. The other 3 cylinders are well within spec of one another, and they were higher than I had assumed they'd be, so I'm happy with those. However, while cranking the car over, cyl 3 would make a popping sound and wouldn't go above 100 psi. Every time yielded the same result with no variations. The only thing is I don't remember if the pop came before or after hitting 100 psi. I failed to note that. The car's power has been pretty good, but still at times it does kind of feel like it falls short. Not always, but on rare occasion. I don't know where this pop noise came from, and have no idea what could be causing it. The only idea I've been able to come up with, is if the pop happens right when it hits 100 psi, could one of the injector seals be popping and then re sealing? However, there's no leaking once it hits 100, and just simply won't go higher either. I'm a little lost on this one. The engine was warm as I had just returned fro ma test drive, and the only thing I forgot to do was open the throttle plate. Injectors were all unplugged and the plug wires were all pulled from the coils.

One thing I'd be curious about is what the compression numbers look like with the engine cold. You may have a tiny crack or HG leak that seals up when the engine is warmed up. And that's how these things start. I think I drove my 1G about 10K miles with a leaking HG before it finally blew. I've actually never done a leakdown test though, and I think in your situation there's a good chance that the leak around cylinder #3 is somewhere else.

Also, I don't know if you have the tools to check fuel trim, but this would tell you if the engine is dumping fuel into the system because of a leaking injector. But I don't think this would cause high coolant temps, though it might. I say, your leak is likely on the exhaust side. I guess the leakdown test is what you need.
 
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Also, I don't know if you have the tools to check fuel trim, but this would tell you if the engine is dumping fuel into the system because of a leaking injector. But I don't think this would cause high coolant temps, though it might. I say, your leak is likely on the exhaust side. I guess the leakdown test is what you need.

I don't have specific tools for fuel trim that I'm aware of. I know my scanner does monitor, but not sure if it's the right checks. This is all live data after arriving at work tonight.

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The IAT says 71*, but the outside temp was about 42*

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This is the "home" screen for my scanner. I would guess the top left corner icon represents a fuel related option, but not sure.

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This is the screen that comes up after selecting that fuel icon, but I haven't gone further because I don't know what it does, and don't want to mess up my car if it has that kind of capability. I still have all the paperwork that came with the scanner when I bough it, but there's nothing about how to use the scanner. I was disappointed.

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I don't have specific tools for fuel trim that I'm aware of. I know my scanner does monitor, but not sure if it's the right checks. This is all live data after arriving at work tonight.

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Looks like you have fuel trim in your scanner: LONGFT1(%) +5.4. Your engine is running a bit lean, but within limits. Not enough to suggest a lot of fuel dumping by the ECU. If you're interested you could watch it in relation to ECT (210 degrees) and see what it does at idle.

I would guess, your #3 compression leak is somewhere else besides the injector.
 
Good to know, thanks for the diagnosis :thumb: What would it display if running in the preferred range? When running rich?

I know running rich will consume more fuel, but what about running lean? I'm getting 15-17 miles per gallon at the moment based on how many miles I've gone between fill ups. I haven't had the chance to look in the tank yet, but I finally got both days off this weekend and can dig into my car for some answers.
 
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I would do a leakdown test, but I currently have no access to a compressor. Until I can find out if a friend has one, I can't do this test.

EDIT:
I actually just found this from someone having the same issue:

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I had considered the possibility of a bad o-ring at the base of my tester giving a bda seal, but I didn't have any spare O-rings with me. I also dismissed this idea since I would think if that o-ring were bad, it should do this on all cylinders instead of only one, and always the same one. My tester sorta swivels a bit at the bottom to prevent over tightening as well (if that's important to know). I'll give it another try this weekend since I finally have 2 days off in a row for a change. Will throw a new o-ring on to see if anything changes.
 
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You are right, which is why I'm limiting the guesses to testing for now and not part replacement (except the o-ring on my tool). I know the value of a leakdown and will do it, just once I get to. I gotta check with a friend today who I think does have a personal compressor.
 
I would guess, your #3 compression leak is somewhere else besides the injector.
An injector or injector seal will not affect the compression in the combustion chamber on a compression test. wanted to clarify this in case somebody else is searching, so they don't get Miss information.

Good to know, thanks for the diagnosis :thumb: What would it display if running in the preferred range? When running rich?

https://www.ecmtuning.com/wiki/fueltrimupdatepoints
https://www.dsmfaq.com/q/113

I would do a leakdown test, but I currently have no access to a compressor. Until I can find out if a friend has one, I can't do this test.

Go to an equipment rental shop. If you have those around where you are. It's like 10 to $20 a day to rent a portable compressor.
 
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when in dire straights, I've done a "leakdown" test with a bike pump, a drilled out spark plug, lots of hose clamps, and my ears. It was super super ghetto, but it let me know that the valves were OK and I had a power-cylinder problem. The popping you're describing really has me leaning toward exhaust valve seating problem on cylinder 3, but who knows for sure until you have the data. Let us know what you find.
 

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An injector or injector seal will not affect the compression in the combustion chamber on a compression test. wanted to clarify this in case somebody else is searching, so they don't get Miss information.

Yes I realized this after thinking about Engine Fundamentals 101. There's no way for air to leak out through an injector seal, which would be a fuel and not an air issue. The OP brought it up. There are seals in the intake/ exhaust valves which are probably old.


The value in LONGFT1(%) would be negative if running rich. I don't think he has the ability to alter fuel trims through Eprom ECU / wideband sensor, it's just the standard fuel trim data that gets put out in OBDII systems and read by scanners. You could also look at SHORTFT1 which should be fluctuating much more than LONGFT1.
 
Yes I realized this after thinking about Engine Fundamentals 101. There's no way for air to leak out through an injector seal, which would be a fuel and not an air issue. The OP brought it up. There are seals in the intake/ exhaust valves which are probably old.



The value in LONGFT1(%) would be negative if running rich. I don't think he has the ability to alter fuel trims through Eprom ECU / wideband sensor, it's just the standard fuel trim data that gets put out in OBDII systems and read by scanners. You could also look at SHORTFT1 which should be fluctuating much more than LONGFT1.

By Seals you mean valve seats not Seals because valve seals do not affect compression results either. And the article written by ECM tuning explains exactly how short term fuel trims and long-term fuel trims work. His question was "What would it display if running in the preferred range?" That link that I posted will tell him the exact answer to his question. There is no information in that article that refers to altering base fuel maps or timing maps. Please stop trying to teach people stuff that you don't have a lot of experience with. I'm trying to be polite.
 
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