Advice from engine builders/installers. Low compression in #5 cylinder.

I purchased this truck with a 3.4L in July 2016 with 174088 miles. One of the things I quickly noticed was how dirty the intake tube was after the air filter. Turns out there were small pebbles at the bottom of the air box preventing the air filter from sealing completely and letting dust to leak by. The PO said he used the truck as a SCORE recovery vehicle for motorcycles and spent a lot of time off road in Baja.

I removed the pebbles and did a leak down and compression test later on all the cylinders. At 176198 miles all the cylinders looked real good (about 6% leak down and 190 PSI) except for the #5 cylinder. It's leak down was 25% and 180 PSI compression. Not great but within the 15 PSI variance specified in the service manual.

So yesterday I was kind of bored and retested the # 5 cylinder. The leak down is now 34% and compression 155 PSI. I put a little oil in the cylinder and the compression jumped up to 180 PSI but leak down stayed the same. I would say the rings are shot and need replacing. The #5 cylinder has degraded quite a bit in just 8266 miles and little over 2 years. May also explain the fairly poor mileage I get at 16 MPG.

I am contemplating doing a cheap as possible ring job. The engine runs great aside from the MPG. The oil pressure when hot is 12 PSI at idle and 64 PSI at 3K RPM. The engine uses no oil between changes. The #5 spark plug is very clean and happens to be 2 cooler because of the supercharger. When I did head gaskets because of a external coolant leak all the cylinders including #5 had no scoring at all and the honing cross hatch was still clearly visible.

A few questions I have are:
Is it at all possible the replace the rings the engine still in the truck? Looks to be difficult to remove the oil pan and torque everything up with the front diff in the way.

What would you guys do? Wait till compression get to the 145 PSI minimum? Or wait till it starts to use or burn oil? Pull the engine and re-ring all the pistons of just #5? Or get a Yotamasters short block? Not really interested in a salvage or Japanese engine. Don't like the chance of getting a worst engine than I have.

Thanks

 

When you did leak down was there any pressure going pass valves?

 

How much money do you have? If it were me based on what you said I would drive it while saving up and keeping in mind that the engine may need replaced down the road. You could run some thicker oil for the time being. If money isn't a factor I would get a long block and build that on the side as much as possible until you're ready to swap everything over from the old engine. I know people who have had good luck with used engines from Japan, but also people who have had problems getting a worse engine. That said a lot of the importers will give a short warranty for issues like that. I agree with you though not very comforting even with a warranty.

 

I'm pretty sure the valves are okay but not absolutely sure. When doing the HG I had the heads steam cleaned and pressure tested. The shop said both heads tested good. But when leak down testing using a stethoscope I can hear some hissing in the throttle body, a little in the exhaust pipe and plenty in the oil filler cap.
Not sure what to make of that.

Money is an issue unfortunately. I purchase the truck from a dealer for about 14K. I have about that much in mods and refurbishing (The TW effect LoL). The house needs a drive way and has termite issues.

 

Yeah I hear you on the TW effect. What oil are you running now? I would run it as is if there are no other issues. Save what you can for a rebuild or lower mileage engine.

 

Run it, fuel mileage isn't out of range and you never know what else could happen before the engine wears completely out. It's an old truck being driven in one of the most congested areas of the country. Sorry but that's a truth.

 

I'm running AMSOIL 5w30. I was hoping a high quality oil would slow or stop the compression loss. Did not seem to help.

I'm thinking I'll go with you two's advice. Run it till there is some sort of drivablity issue. I saw @kuntry09 post about getting an error code and a very low #5 compression. We both seem to being having the same problem aside from I am not get a code yet. Low #5 compression that oil in the cylinder brings back up. Sure sounds like piston rings to me.

Makes me wonder if it is just coincidence that Kuntry09 and my #5 cylinder is failing or is there some other issue with this engine. Sure would like to hear from other members with high mileage engines what there compression numbers are? Maybe just cylinders #2 and #5 because they are the easiest to get at. Would be great to hear from @1997tacomav6. He has a well maintained, high power, high mileage engine.

 

I think it’s coincidence and just plain bad luck on my part. My first motor went 340k miles, and was still running when it was pulled. Granted, wasn’t running the best, but considering it had a head gasket problem and still running I thought was impressive. At this point I don’t know what I’m going to do. I suppose a leak down test is next, (not so sure if I’m going to tackle that myself) but I already have too much money into the motor swap and 4wheeldrive swap to let the motor get too far gone to be able to fix for a reasonable amount...I would hope anyway

For what it’s worth, I show around 15mpg between highway and city driving. That’s pushing 285/75/16. Open highway I can hit 18, but the average comes out to 15

 

Try switching to 10w-30

 

My 01 3.4 has 503,000 on the original engine. I did a compression test about 500 miles ago. They ranged from 110-130 psi. My #5 was high @145 psi, but I have a leaky valve cover gasket that made it test wet (could be worn rings, will dive into it soon). #6 was the lowest @110 psi. Cylinders 1-4 were 125-130 psi. I know its below recommended specs....but the truck still pulls great up here at 6000-8000 ft (gets up to speed with normal traffic and can keep up), and gets around 17 mpg, that's with armor and 33s. I plan on putting in a low mileage engine once the head gaskets go, probably going to source one from our local Toyota junkyard.

 

for a very small amount of $ you can buy a bore scope that plugs into your smart phone... pull #5 plug and put her on BDC and take a look and see if the cylinder is scratched. If it isnt scratched then look at the top of the piston for impressions. Very, very small possibility that a pebble pinched the top ring.

 

QUOTE="Glamisman, post: 18872263, member: 200436"]for a very small amount of $ you can buy a bore scope that plugs into your smart phone... pull #5 plug and put her on BDC and take a look and see if the cylinder is scratched. If it isnt scratched then look at the top of the piston for impressions. Very, very small possibility that a pebble pinched the top ring.[/QUOTE]

Yes, bore scopes are very handy especially when your bro already has one. The top pics are of the #5 piston top. The lower right pics had me convinced there was a crack between the intake and exhaust valves. Turns out there was no crack and the head was fine.


Here is the cylinder wall.

Wow...that is incredibly low compression. That's about the pressure of my lawnmower LoL. Do you need to add oil between changes?
Thanks for posting. I guess I don't have to worry about a sudden failure.

I wonder if there is some dynamic sealing going on. Maybe since the running rpm is so much higher than starting RPM, there is less time for leakage and the compression is higher while running. Just speculation on my part.

 

I do add oil. Maybe 0.5-1qt. every 3000 or thereabouts. I'm do for an oil change and will tackle the valve covers too, I just wonder how much oil is seeping through there vs losing it in other places.
And yes....very low compression. Sure doesn't act like it!

 

if that is the cross-hatch pattern throughout the bore, I see no problems. In re-reading your first post you added oil and the leak down didnt change, to me that indicates that there is an issue with valve(s). Re-run the leak down test and take the tube off the throttle body and crack it and listen, then do the same to the exhaust pipe. Either way the head is going to have to come off.

 

Yes, when adding oil the leak down did not change and stayed at 34%. But the oil did raise the compression test from 155 to 180 PSI. Maybe oil has no effect on leak down but works with a compression test because the pistons are sliding in the bore. I understand that if the valves are leaking or there's a crack in the head then the compression will change very little when adding oil to the cylinder but I maybe wrong???