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*Updated: 01/17/2022 6:54PM PST

VG30E Compression Test Questions


Albeitt
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Hi, my 1993 SE starts and runs, but has white smoke and coolant coming from the exhaust. I'm a newbie, so I have a few questions.

 

Since coolant is being burnt, I'm apprehensive about letting it run and warm up for a compression test. Is this concern valid? Or would it be okay and probably more accurate to let it warm up for about 10 minutes before I perform a compression test? Also, would it better to let it run for maybe a little less time than the recommended 10 minutes?

 

I've also heard things about the #6 spark plug. Apparently it's a pain in the ass to remove. Would it be easier to just run the test with a cold engine?

 

It also sounds like it's misfiring slightly, so that's another concern I have with letting it run for too long. I'd rather not create a whole other problem.

 

Thanks!

Edited by Albeitt
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I would let the truck warm up and then do the compression test, the coolant is most likely a head gasket so the compression test should tell you which one, the compression-hold test is the one you really want to use with a possible head gasket failure. It's a compression test done over time watching the _rate_ of pressure loss over a 5 minute window for each cylinder. All of the cylinders should be close except for one or two, depending on the specific type of failure

 

You can do the compression test cold as well, I did mine cold and still had 169psi as the lowest reading of the six with the rest all hanging around the 175 to 180 psi mark. The #6  is a pain but if you have the original tool bag it has the correct length tool to reach all the way down to the #6.... there is a combination of regular extensions that will get you the same thing but hella difficult to get the plug back into its spot with the damn wobbly extensions. Take the hood off so you can at least have some space to work. It will make your life _much_ better!!

 

As for the misfire, there are too many things to list to even begin what it _could_ be, but start with new plugs, wires, cap and rotor. That will eliminate the most irritating and easy things that MAY be slightly wrong / cracked/ broken /tracked or otherwise out of specification. After that we have lists and lists of things that can cause a misfire, but they cost more money!

 

 

That's all I got for ya!

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23 minutes ago, SpecialWarr said:

I would let the truck warm up and then do the compression test, the coolant is most likely a head gasket so the compression test should tell you which one, the compression-hold test is the one you really want to use with a possible head gasket failure. It's a compression test done over time watching the _rate_ of pressure loss over a 5 minute window for each cylinder. All of the cylinders should be close except for one or two, depending on the specific type of failure

 

You can do the compression test cold as well, I did mine cold and still had 169psi as the lowest reading of the six with the rest all hanging around the 175 to 180 psi mark. The #6  is a pain but if you have the original tool bag it has the correct length tool to reach all the way down to the #6.... there is a combination of regular extensions that will get you the same thing but hella difficult to get the plug back into its spot with the damn wobbly extensions. Take the hood off so you can at least have some space to work. It will make your life _much_ better!!

 

As for the misfire, there are too many things to list to even begin what it _could_ be, but start with new plugs, wires, cap and rotor. That will eliminate the most irritating and easy things that MAY be slightly wrong / cracked/ broken /tracked or otherwise out of specification. After that we have lists and lists of things that can cause a misfire, but they cost more money!

 

 

That's all I got for ya!

Yeah there's sweet smelling white smoke and water in the exhaust, so definitely a coolant leak into the cylinder. Since there's no coolant in the oil, is it possible that it's a intake manifold gasket? Maybe there's cool leaking from that into the intake? 

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If you are burning coolant, that is the likely misfire as well. Easiest way to see which cylinder it is, is to pull and examine the plugs. The one(s) burning coolant will be very clean while the rest will have some deposits on them. 

Most likely leak for coolant into the cylinder is head gasket failure. The intake manifolds on the VG engines are dry, meaning no coolant passages in them. Other coolant into the cylinder leak possibilities are a cracked head or cylinder. Both of these are very rare issues with this engine, and a cracked cylinder will usually have coolant in the oil as well. 

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I wouldn't bother warming it up. Either the plugs will tell you where your problem is or the compression test should work well enough to tell you where the problem is. You're not looking for good numbers across the board, you're just looking for the one weak hole. #6 isn't fun, but it's mostly a matter of it being hard to see. If you've got some extenders and a U joint, or the stock toolkit from under the back seat, it's not too bad to remove. The compression tester took some screwing around before it would thread, partially because it was bent, partially because I wasn't holding it at the right angle. Blow out around the plugs before removing them if you've got compressed air, they tend to accumulate dirt and rodent crap in the wells.

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14 hours ago, SpecialWarr said:

the compression-hold test is the one you really want to use with a possible head gasket failure. It's a compression test done over time watching the _rate_ of pressure loss over a 5 minute window for each cylinder.

Not sure i understand this test, The compression tester has a schrader valve in the end of it and should not leak once it is up to pressure.

 

8 hours ago, Mr_Reverse said:

The intake manifolds on the VG engines are dry, meaning no coolant passages in them.

Not true, The lower intake manifold does have coolant passages so that could be your failure, I just went to the garage to double check because I couldn't recall.

 

4 hours ago, Slartibartfast said:

I wouldn't bother warming it up. Either the plugs will tell you where your problem is or the compression test should work well enough to tell you where the problem is. You're not looking for good numbers across the board, you're just looking for the one weak hole. #6 isn't fun, but it's mostly a matter of it being hard to see. If you've got some extenders and a U joint, or the stock toolkit from under the back seat, it's not too bad to remove. The compression tester took some screwing around before it would thread, partially because it was bent, partially because I wasn't holding it at the right angle. Blow out around the plugs before removing them if you've got compressed air, they tend to accumulate dirt and rodent crap in the wells.

+1 on all this , Run a regular compression test cold, if you find the offending cylinder with the steam cleaned plug and compression is good then remove the schrader valve from your compression adapter and apply compressed air to that hole, see if you have bubbles in the coolant. if you do than it is a headgasket failure. If no bubbles than there is a possibility of the lower intake manifold leaking, which would be much easier to do.

 

And make sure you do a compression test on all 6, even if you find your coolant leaking cylinder earlier, would be a shame to do a bunch of work like manifold gaskets, plugs, wires, cap, rotor, headgasket etc, Only to find our you have a bad intake valve. I learned the hard way on my 91.

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8 hours ago, snowboard419 said:

Not sure i understand this test, The compression tester has a schrader valve in the end of it and should not leak once it is up to pressure.

 

Not true, The lower intake manifold does have coolant passages so that could be your failure, I just went to the garage to double check because I couldn't recall.

 

+1 on all this , Run a regular compression test cold, if you find the offending cylinder with the steam cleaned plug and compression is good then remove the schrader valve from your compression adapter and apply compressed air to that hole, see if you have bubbles in the coolant. if you do than it is a headgasket failure. If no bubbles than there is a possibility of the lower intake manifold leaking, which would be much easier to do.

 

And make sure you do a compression test on all 6, even if you find your coolant leaking cylinder earlier, would be a shame to do a bunch of work like manifold gaskets, plugs, wires, cap, rotor, headgasket etc, Only to find our you have a bad intake valve. I learned the hard way on my 91.

Okay, thank you for the advice! This forum is definitely worth it.

 

I'm really hoping it's just an intake manifold gasket or something else in the intake assembly. I know mechanical basics and how an engine runs in theory, but something like a head gasket replacement is a little beyond my comfort zone for the time being. I got my Pathfinder for super cheap and under the notion that it'd be a learning opportunity, so it's not a huge loss if I can't reasonably fix it, as I definitely cannot afford a shop to do the work.

 

Thanks!

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The test of leakage over time is a leak down test. It just shows that something is leaking, could be valves, head gasket, cracked heads, cracked cylinders, bad rings. 

As said, when you put the cylinder under test to TDC on the compression stroke, and apply compressed air through the spark plug hole, where it comes out is a good indicator of the problem. If it is leaking into the cooling system, odds are good it is head gasket, though cylinder and head cracks will do it also. Leaking out the intake indicates intake valve leaking. Exhaust, the same but exhaust valve. Crankcase is usually a ring problem. 

 

From the description and what I have seen with Nissan engines, my money is on head gasket. 

From my personal experience, if it is a bad gasket, I would be more likely to get a good engine from a salvage yard and just swap. With an engine with more than 120k miles, just doing a head gasket is rolling the dice. And my experience has not been great there. Also found it expensive to properly rebuild a Nissan engine. $200+ in just head bolts. My local yards sell engines for that.

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Well the way i look at it its already f*cked up so worst case its still f#ed after your done. Best case you learn something and have a sweet ride after your done. There is a link to the factory service manual somewhere on here, if you find it download it and save it as they are not always around. I have rebuilt a few VG's in the last couple years so feel free to hit me up with any questions. Dont let the head gasket intimidate you its all nuts and bolts just takes proper procedure. If you end up pulling the heads ABSOLUTELY replace the exhaust studs as they are very prone to breaking, and you will more than likely break a few or more removing the exhaust manifolds. If you have the time and tools available drill and tap the holes out for larger 10mm studs found on the vg33. Dont forget timing belt, water pump, and all that. Also genuine Nissan Exhaust gaskets are far superior to anything aftermarket i have seen but they are a bit spendy. And there is a place you can send off your fuel injectors to have cleaned and tested for something like 20 bucks each, cheap insurance if you ask me. 

 

15 minutes ago, Mr_Reverse said:

. Also found it expensive to properly rebuild a Nissan engine. $200+ in just head bolts. My local yards sell engines for that.

+1 on this if your on a budget. I have easily  spent north of 500 to 700 to do a budget rebuild but that included a bunch of bottom end stuff also. Hop on rock auto and price out everything you need to see what direction to go. I have personally measured and reused headbolts on my rebuilds with no issues thus far, Not saying its right but i am cheap. 

 

Better yet swap a junk yard motor in and take your time to rebuild the one in the truck now if you find any major problems.

 

 

Edited by snowboard419
typo
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+1 to the leakdown test. You can also get test strips to see if the coolant has exhaust gas in it. If the head gasket is bad enough, you can pull the radiator cap, start the engine, and look for exhaust bubbling out of the coolant.

 

You can get a '94/'95 manual from Nicoclub that'll cover most things, but the '90 manual from cardiagn.com covers the square-dash trucks better. That's my go-to for my '93. The engine rebuild info should be about the same, though.

 

The first time I did head gaskets was on a friend's POS Astro van, which probably had 200k on it at the time. It was a miserable job, mostly because there was a van wrapped around the engine and we were working outside in the snow. Other than that, it really wasn't too bad of a job. I haven't been that far into mine. The timing belt will make the job a little more complicated (versus the pushrod 4.3 in the van), but the access will be much easier! I removed the hood before I took my intake manifold apart and would recommend doing the same, it doesn't take long and it makes the engine bay much nicer to work in.

 

If you decide to go the engine replacement route, and don't mind a little extra screwing around, you could do the 3.3 swap while you've got it all torn up anyway.

 

 

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