DieterMahlein

Please bear with me, it's my first post, and I'm not much of a mechanic.
I'm the original owner of my 94 Toyota pickup 4x4 with 3VZ engine. I have a small camper on it, and I've been traveling with it through South America. It has 196k miles on it.
A year ago, it overheated and blew a head gasket in southern Chile. We had that repaired at a Toyota dealership; they did not replace the valves.
About 4000 miles later, a mechanic noticed that cylinder #6 had almost no compression, because the exhaust valve had no clearance to close. They machined down the spacer disk, which restored proper compression.
Now, about another 6000 miles later, compression in this cylinder is again down to 50 psi, while the other five read between 130 and 150. The mechanic advises against another adjustment and recommends replacing the valves; this means taking the engine apart again. He says the valves should have been replaced when the engine was apart in Chile a year ago.
Do you agree? I welcome comments and advice. Thanks!

blueknob

anytime i've done a head gasket, my machinist just checked to make sure the valves were sealing, i wouldn't replace the valves unless there was a problem, the clearances should maybe be checked, low compression could be rings, valves or even a headgasket

thook

It sounds like you may have a burned valve on cylinder 6....which may be why the mechanic suggested replacing the valves. Now, did the mechanic recommend replacing all valves or just on the problematic cylinder?

Also, I'd be curious to know if the gasket that blew was on the very same cylinder head as #6. Could be why you blew the gasket in the first place.

RJR

When the valve and or seat wear away enough so that the valve won't full close, the valve starts to burn due to poor cooling and to a blast of hot exhaust gases going past it. That's what happened the first time you had low compression. By changing the shim thickness, you got the valve to seat enough to get the compression back up temporarily, but you still had the partially burned valve, which likely has still been leaking slightly. As before, the hot gases leaking past the valve have now burned it even worse. I suspect your mechanic is correct - you need a new valve.

When the valves are working correctly, the fire in the cylinder is largely burned out and the exhaust gases have cooled to 1500-1700 degrees before the valve opens. When a valve leaks, the gases blowing by are at the full combustion temperature, which is probably upwards of 5000 degrees. Valves don't last long in that environment.

DieterMahlein

Thanks guys, I do appreciate your responses!
The head gasket blew only at cylinder #1 on the 1-3-5 side, the 2-4-6 side stayed intact.
I think we will be replacing all valves; shouldn't we? What about guides and seats? Anything else, while we're in there?
Thanks again!

Budman1

I'm concerned about higher compression affecting the bottom end if you do a complete valve replacement. Can a Toyota engine with 196k miles sustain the higher compression of new valves?

bud

thook

These guys raise some good points. You did have some lower compression readings. So, if you're going to get new valves, the rings will have wear and are a factor. But, look, you don't have to get all new valves. When I rebuilt my 22re....due to timing chain failure and six bent valves.....I had the head rebuilt, but two of the valves were still reusable. They just ground them. The guides, seats, seals....all that stuff gets reworked when the head/s are serviced as a rule. As for the bottom, I did the work myself. I inspected everything for tolerances....like bearings and cylinders....and determined even though everything was still in spec, since the engine was near 200k miles, I'd go ahead put in new bearings and rings. But, the camshaft and crankshaft I left alone as there was need for machining. And, I just honed the cylinders. And, of course, I put in a new timing chain kit w/ a water pump and oil pump (the old oil pump was fine, but the kit came with a new one so I just put it in). This might be considered a "partial rebuild", but it brought compression up to optimal, I won't have to mess with it again in another 100k (or so) due to rings or bearings (low oil pressure), and now it runs like a champ. You hauling a camper and traveling would do well to look at doing the same.

As for anything else, perhaps your timing belt, pulleys (some of them tend to dry out at around 200k), and water pump. When was the last time those were serviced? Then, there's the wiring/connector from the knock sensor under the lower intake manifold. Those tend to get brittle at high mileage and can cause k.s. signal problems and you can only get to it when the top end is off. What else? PCV valve; cheap and everything will be apart to get to it. Clean the throttle body and maybe a fresh air filter. Plugs, ignition cables, distributor cap, fuel filter? How old are those? These latter items are general maintenance stuff, but still worth looking at if it's been a while. Oh yeah...and injectors! Have yours ever been serviced? The pintle style injectors these engines use are more prone to clogging (than newer style) and sometimes solenoid malfunction around 100k. More so the clogging, for sure, but everything will be apart and that would be a very good idea to go ahead and have them cleaned. And, of course, have all the vacuum lines and coolant hoses thoroughly examined for deterioration.

Anything else, guys?

RJR

I've heard this concern mentioned before, but I'm not sure it holds water. Valves pretty much either seal or they don't. As soon as they start leaking they get destroyed in a fairly short time (see my earlier post as to why). Which is another way of saying that most engines that are running well at 200,000+ miles still have good valve sealing even with the old valves, and replacing the valves isn't going to significantly increase the compression except in the cylinders where a valve is burned, and then that cylinder will simply go up to match the rest of the cylinders.

Compression loss due to valves is usually a step function. It goes from normal to almost zero in a pretty short time period. Compression loss due to ring and cylinder wear is a more gradual decline as things wear and blow-by increases. Replacing the valves won't boost ring related compression back up.

Now, if in the process of doing your valve job you mill the head significantly, you will increase compression over factory specs and will indeed increase the stress on the bottom end. For example, shaving .020 off the head will increase the compression ratio on a 3vze from 9.0:1 to about 9.5:1. Could be significant.

thook

That raises a point I'd thought about when he mentioned the blown gasket. I'd wondered if the head was checked for warping and/or if the head was subsequently milled at that time. Deiter???

ps. Deiter....get them to use new head bolts. The OEM bolts are torque to yield. It's been stated they're not reusable, though I've known people to reuse them. And, sometimes the bolts have failed and sometimes not. I think Ted over at engbldr (a popular toyota parts supplier and service shop around here) even had a video at one time of the bolt heads just popping off while the engine was sitting the shop. Anyhow, worth not messing around with.

Ron....would new valves increase blow by on older rings?

RJR

I wouldn't normally think so, because again, valves usually seal very well until they get a small leak, and then they go south quickly. You likely are not driving around with a bunch of "slightly leaky" valves - there really isn't such a thing as a long term "slow leak" on a valve. It's like a hole in an earthen dam. Once water starts flowing through it, it gets bigger in a hurry.

BTW, this discussion is assuming we're talking about exhaust valves. Intake valves lead a much more sheltered life.

Budman1

......."Ron....would new valves increase blow by on older rings?".......

Ron, my point, and this question directs the concern to how the existing piston rings will react to new valves that will create higher compression numbers.....I'm not saying the 176k engine will be weak to higher compression, rather asking y'all with more experience with Toyota 3vz.



bud

RJR

Apparently I'm not communicating clearly here. My point is that, valves don't cause compression to slowly decrease over the life of the engine. (worn rings will do that.) They don't "gradually" get worse over 50-100,000 miles. They work like new ones until they start leaking, and then very quickly fail and compression drops to near zero. The reason is that they simply can't survive long while leaking - the hot gases and poor cooling quickly eat them up. Replacing the valves simply gets your compression back to where it was before one or more of the valves started leaking. So if the bottom end was good to go before the valves started leaking, it will still be good to go.

RJR

(Can't seem to edit posts in this forum anymore, so I'll reply to my own).
If you've been running at 75% compression for the last 50,000 miles, the problem is probably not valves, and replacing them won't restore the compression to 100%. On the other hand, if you have 5 cylinders at 75% compression and one that has suddenly (over the last 4-5000 miles) dropped to 20%, just doing a complete valve job (or replacing the valve on the bad cylinder, either way) will likely restore all of them to 75%. You'll have to work on something else to get back to 100%.

DieterMahlein

Thank you all once again, I'm grateful!
A little more info: After the gasket blew, the heads were checked, found well, and machined down 1/10 mm.
Timing belt is new. Knock sensor, distributor cap, rotor, cables, and coil will be replaced; I assume original Toyota parts are best here. Heading the advice, we'll also replace water and oil pumps; don't know if they've been touched ever...
Yes, this is the exhaust valve in #6 only... so will replacing just it be fine?
Probably forgot something; will read your reponses again.
Thanks much!

DieterMahlein

What's 100% compression? We're getting 130-150 psi on 1-5, but only 50 psi on #6. Thanks!

thook

re: exhaust valves...right. but, dieter was considering all new ones. nevertheless, i see exactly what you're saying.

now, valves aside, he does have low compression readings on come cylinders. i do suspect rings myself since i assume valve clearances were fine except on 6 when that was discovered.

thook

he means what would be ideal or like new....which would be at 170psi, iirc. 150 is quite acceptable, but 130 is kinda low.

thook

your call, but at 196k i'd want new valve seals since the head is being serviced, anyway. and, since you're having to do one head why not do the other. still doesn't mean you need all new valves. like ron is saying, it's the exhaust valves that see the most wear. so, unless they find irreparable damage to any others, just have them refreshed and replaced the burned one....if it is in fact burned. i'm just making an educated guess on that, afterall.

also, toyota parts....yeah, if you've got the money, toyota is the quality.

millball

Compression test numbers can be somewhat misleading, unless reference is made to the altitude above sea level where the test was made.

The 170psi number is ordinarily only possible to achieve very near sea level.

thook

I've read otherwise, but let's that is absolutely true. The bigger factor is compression variance between cylinders, no? Isn't it like no more than a 15psi difference that's considered proper? I think it is. Atleast, to have a well running engine.