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Opinions wanted : Higher mileage 1st Gen Tacoma engine woes


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305k on a 3.4 5ve-fe v6

Long story short, I was replacing the intake manifold gasket and screwed up a broken bolt extraction so I was faced with removing one of the head(s).  Considering the miles I decided to do a compression and leak down test while the spark plugs holes were easily accessible. 4 of the 6 cylinders are 60psi and the other 2 are at 120.  Leak down test showed cylinders with the lowest psi were definitely leaking through the intake valves and the other two inconclusively through the coolant system, it was hard to tell with  the last two since I had drained system for the intake manifold replacement. 

 

Besides average mpg being down to 17mpg from the normal 20-22 and maybe a slight reduction in power there was no indication the engine was in this poor of condition.  Also I recently bumped up tire size so I dont think these two "symptoms" alone are out of the ordinary. I'm legitimately baffled that the engine was running at all with the results of the test.  

 

Here's where I'd like any insight available:

I've spoken to a few different Toyota/truck mechanics to get some feedback on if I was performing the tests correctly and about how to move forward.  They unilaterally all said that if they were dealing with an engine the had the same symptoms I was describing with as high mileage as my block has they wouldn't even think about replacing the heads alone. It would automatically be a full engine replacement. I'd really appreciate hearing any opinions or experiences that could help with the decision. Also, can anyone tell me how 4 intake valves get burned on a really well cared for truck with a famously hearty and reliable engine platform, while at this same time showing no signs of it???  It was religiously serviced at the dealer since it was new until I bought it with 270k and I am very diligent about servicing it. 

Edited by TheBirdistheWord
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Get a second opinion with a different compression tester. 60 psi is basically nothing, how would it run and get that good of mileage? It would make more sense to be 120 X4 and 150 X2.

 

1 3 5

2 4 6

What cylinders are low? If all low cylinders are adjacent cylinders like 1-3 , 3-5, 2-4 or 4-6 possibly the head gaskets are blown/blowing between them. If 3 and 4 are the high compression numbers then not this.

 

Intake valves won't be burned. Exhaust valves can burn if hot exhaust can erode a path out across the seat. Intakes can wear the seats or become pitted. 

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8 hours ago, Draker said:

Did you already pull the heads to confirm the burnt valves? It does seem pretty odd. I’ve got the same motor in my tacoma. 203k at the moment. 

Not yet, one of them has to come off anyway because of the aforementioned FUBARd intake manifold bolt extraction. Lost the wind in my sails after the tests so I stopped putting time into it until I decided what direction to go since as far as I can tell the engine would be easier to pull with the heads installed

8 hours ago, Draker said:

You can get a usb bore scope on eBay for like $20. Connects to your phone via Bluetooth.

I'll look into this, but probably will just pull heads if there's a chance it's salvageable

6 hours ago, datzenmike said:

Get a second opinion with a different compression tester. 60 psi is basically nothing, how would it run and get that good of mileage? It would make more sense to be 120 X4 and 150 X2.

 

1 3 5

2 4 6

What cylinders are low? If all low cylinders are adjacent cylinders like 1-3 , 3-5, 2-4 or 4-6 possibly the head gaskets are blown/blowing between them. If 3 and 4 are the high compression numbers then not this.

 

Intake valves won't be burned. Exhaust valves can burn if hot exhaust can erode a path out across the seat. Intakes can wear the seats or become pitted. 

I had the same thought since the compression tester I initially used was rented from the parts store. Tried two different brand new testers and got the exact same results. It's been a few months so I'm a little hazy on which cylinder s were highest psi, but I think it was 3/5 and 2/4 at 60psi 1/6 at 120.  The second two tests I wrapped plumbers tape around the threads to make sure the tester in the spark plug threads wasn't causing a false reading. All of the plugs were removed during every test as I was advised keeping them in had potential to give me false positives if the head gasket had failed between two cylinders.  Leak down test had an audible leak through the intake valves, and it was easy to tell since the manifold was off. 

 

I triple checked I was at TDC on each cylinder because I was convinced I was doing something wrong considering the results

 

I suppose "burned" is the improper term, there is a massive amount of carbon build up on the TB, runners, manifold, and intake valves.  I suspect the pcv valve failed some time back but again don't think this could be the sole cause.  One thing I am almost certain of is it's NOT the bottom end. Put new plugs In about 5k miles ago and they looked brand new when I removed them for this job. 

Edited by TheBirdistheWord
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Does this engine have variable cam timing????

 

 

Warm it up and do the test again but after each one squirt some oil in and immediately re-test. Compare the two readings. If it went up significantly like 10 or more pounds, the rings are bad. If only a few PSI then not.

 

I've never done this with all the plugs out. The engine will turn faster with them out but compression with them in is the same as when starting so??? I don't think it maters much as long as you do it the same for comparison.

 

If the low cylinders are side by side take a really good look at the head gaskets you take off where the cylinders almost touch.

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47 minutes ago, datzenmike said:

Does this engine have variable cam timing????

 

 

Warm it up and do the test again but after each one squirt some oil in and immediately re-test. Compare the two readings. If it went up significantly like 10 or more pounds, the rings are bad. If only a few PSI then not.

 

I've never done this with all the plugs out. The engine will turn faster with them out but compression with them in is the same as when starting so??? I don't think it maters much as long as you do it the same for comparison.

 

If the low cylinders are side by side take a really good look at the head gaskets you take off where the cylinders almost touch.

Not sure about the variable cam timing, but I can see why you ask. If I didn't disable it before test there's a possibility it could have adversely effected it. 

 

Well if it was the rings I'm certain I would have seen at least some soot on the plugs when I removed them, they literally look brand new. 

I guess I'll have to tear the engine down more to get a conclusive answer. 

Edited by TheBirdistheWord
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You can always do a leak down test to determine where you are losing compression.  If it's valves you will hear it blowing out into intake or exhaust.  I built a ghetto one with an air compressor blow valve, a spark plug non fouler, a 12 inch grease gun hose and fittings to mate them all together.  Make sure valves are closed, Twist snugly into spark plug hole, connect air, give it some pressure, then listen at intake, exhaust manifold, and oil filler neck for rushing air.

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So pulling the heads will obviously give me a better idea of what's happening, and I will do that asap. But I'm still interested in opinions about the initial inquiry. Say the heads ARE in fact in need of a rebuild, and the block IS good.  Who here would throw rebuilt/new heads onto a block with 305k vs getting a low mileage/new longblock. 

 

I should mention I already have everything needed to refresh a low mileage engine since I was in the process of doing a major tune up.  Timing belt, water pump/thermostat, and all pulleys were replaced on current engine 10k miles ago as well as everything to re seal the top of the engine in hand. I like the idea of getting used engine because I get all of the accessories as replacements and don't need to provide a core. I even found someone who provides a 5 year 50k warranty on their used engines with up to 80k on them. 

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On a side note, I was confused as to why the various jdm engine suppliers had a hard line on their engine mileage considering the local market wants just as much for engines with double the mileage. So I did some digging and I'm sure some people here already know this but I found out that Japan's inspections are so strict that they frequently decommission cars in the 40-80k mile range for things as insignificant as worn tires. 

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Looks like my employer needs some help at a new facility.  I told him I couldn't commute 2 hours one way In a 50 year old race car and I was able to negotiate getting my engine and a large amount of maintenance items paid for up front to start work at the new facility asap!  

 

About time I got a break......

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