Used to boost a lot, now just boosts a little

[paul vixie]

1983 280ZXT T-5 2+2 stock, ~245K miles. When I got this car it boosted to ~5 with good neck-snap. (Had to replace all vacuum lines etc, it was a barn find, 10 years out of reg.) Eventually I put Marvel Mystery Oil in, thought it was an engine cleaner, but it looks more like STP ("honey"). That weekend my turbo started boosting only as far as ~2, and no neck-snap. I've since checked shaft play (none, and compressor wheel spins freely), waste gate (moves freely, and actuator arm is not too long), manifold leaks (none), and I just tonight cleaned my lifters (I think I had an I5 until tonight, now it's definitely an I6) and cleaned my plugs.

 

I'm looking for other tests I can run or things I can try. I wouldn't mind finding a replacement turbo (note, I want to keep this car stock, so a disco potato isn't in my future, and note, I'll be wimping out and having the exhaust shop do the turbo swap due to the number of studs I expect to break off in the process.) But other than "your compressor wheel isn't spinning as freely as you thought" or "you must still have a vacuum leak somewhere", I am looking for other tests I can run or things I can try. I eventually ran some engine cleaner through, to get the Marvel Mystery Oil out, and I'm running VR1 20/50 ZDDP. Themostat's new, temperatures are good, ECCS harnesses (ECU and engine bay) have all been cleaned well, CHTS is good, timing's good, TPS is good.

 

Help?

[goodoldjam]

Basically everything seems fine and in good shape. No leaks or anything... Hmm. If i can't say it's a vacuum leak or a bad turbo. Then the only answer you will get is something is wrong with your wastegate. When you say it moves freely do you mean you grabbed hold of the arm and moved it?

Edited July 13, 2010 by goodoldjam

[paul vixie]

yes, i disco'd the actuator arm from the wastegate arm and got completely free movement on the arm from full-open to full-shut, with a nice strong clinking sound and a positive all-at-once landing at both ends of travel. of course i didn't see the innards so there could just be a hole in the flapper or pieces missing or it's not seating correctly when closed -- sure feels right though. while i had it all apart i pressure tested the wastegate diaphram, it moves at ~6PSI like it should, and has a good spring inside.

 

i'm starting to wonder if my catalyst is plugged somehow, since i seem to remember hearing more turbo-spool noise when the whole thing was working better. i guess i'll disco below the downpipe/O2 and just play it loud for a bit and see if i can get the old/good boost that way? trouble is, the more i take this exhaust apart, the more busted studs i gotta deal with. anybody got a stock downpipe for sale? the one i've got in now has stripped O2 sensor threads, my "spare" has some busted and stripped/rusted studs stuck in it.

 

my theories are that if i had leaks in the wastegate pressure lines i'd've blown up the engine by now, and if i had leaks in the intake large enough to vent turbo pressure, i would hear that happening, and i would not have such strong compression braking when going downhill in gear. if these theories are of low quality then please please please shoot them down.

[goodoldjam]

I can open my stock gate connected, even for such a weak spring it feels pretty strong.

So it opens fully at 6psi, does it open at much lower pressure? I wasn't thinking of a leak, but I was thinking a weak or broken spring.

I think your theory on a plugged cat is pretty good thing to test and fairly easy.

 

I may have a stock DP ready to go in the next week, that depends on how many studs i snap.

[paul vixie]

the spring is strong. i can force it with a screwdriver while it's attached, and it takes some real effort.

[goodoldjam]

the spring is strong. i can force it with a screwdriver while it's attached, and it takes some real effort.

Well if nothing is leaking and your wastegate seems fine, then i think your left with your plugged cat. That maybe followed with overheating.

Unless something is leaking during boost and you can't hear it.

It's also seems strange that it happened right after the Marvel Mystery Oil mixed with 20/50 but I can't think why it would cause a issue. That mixture probably makes your oil pretty thick though.

[junglist]

Have you checked the j-pipe connector boot or the intake arm to see if they have any rips or tears? Do either of them compress under engine load? You should also check the pop-off/bypass valve on top of the intake manifold to make sure its not stuck open and can open/close on its own. Also, make sure your PCV valve & lines aren't clogged.

[paul vixie]

Unless something is leaking during boost and you can't hear it.

assuming that could happen at such a massive are volume and velocity as the output of a turbo, and assuming that it was somehow a one-way leak such that i could lose boost but still have compression braking... how would i go about testing for it?

 

It's also seems strange that it happened right after the Marvel Mystery Oil

my theory on that was, the turbo shaft had been held in place by oil-jello that the marvel mystery oil broke up. however, i can't imagine a shaft bearing problem that could result from such a sequence that wouldn't also result in a lot of shaft play -- there's none. so, at this stage i'm calling the marvel mystery oil a coincidence.

 

Have you checked the j-pipe connector boot or the intake arm to see if they have any rips or tears? Do either of them compress under engine load?

no rips, no tears, no compressibility with my hands. but wouldn't intake leaks also keep the car from having compression braking?

 

You should also check the pop-off/bypass valve on top of the intake manifold to make sure its not stuck open and can open/close on its own.

i'm able to pull it up but it closes pretty firmly. if it was opening it would be all-at-once and would make the characteristic "pop" "whoosh" sound, i think. but this is worth checking into. i'll try some nylon tie-wraps on it to hold it forced-closed, and see if that changes anything. good idea, thanks.

 

Also, make sure your PCV valve & lines aren't clogged.

i never thought of the pcv. will do, thanks. see also http://www.zcar.com/forums/638061/ecu-wont-go-closed-loop-mode-280zxt which suggests that an EGR diaphram leak is worth visually checking for.

[goodoldjam]

Unless something is leaking during boost and you can't hear it.

assuming that could happen at such a massive are volume and velocity as the output of a turbo, and assuming that it was somehow a one-way leak such that i could lose boost but still have compression braking... how would i go about testing for it?

Compression braking wouldn't be a sign of either a stuck wastegate or a weak spring/low boost pressure. The engine is the only thing under no throttle which effects your compression braking. Remember that your turbo is doing nothing during closed throttle.

 

The system is fairly small most of your testing could involve plugging possible leaks, like the sources junglist suggested.

Edited July 15, 2010 by goodoldjam

[paul vixie]

The system is fairly small most of your testing could involve plugging possible leaks, like the sources junglist suggested.

 

ok so there were leaks but not in the vacuum side. a new PCV valve didn't solve it, and there were no leaks in the PCV hose. i ended up having three problems, and have fixed two and worked around the third. i still need a little advice if i havn't bored everybody to tears yet.

 

1. i had a boost leak at the coupler (output side of turbo). the rubber was cracked in a way that only leaked under pressure. at ~250K miles on factory rubber, it makes sense that it would hold pressure for a while and then disintegrate, the car had been parked in a (hot) field for ~10 years before i bought it. the fix was to buy a three foot chunk of outrageously expensive 2-inch radiator hose and replace the rubber in the coupler. my son (ineptitude01) also took a few inches for his 82 ZXT. if anybody else wants a chunk to replace the rubber in their coupler, send me a self addressed return mailer and it's yours. with a new coupler, i got my boost gauge up to ~200mm/hg. still well short of the red zone but higher than it had been in months. so, a partial fix.

 

2. the waste gate spring is weak. it tested at 8psi with a bicycle pump, so i'd rule it out, but thanks to goodoldjam and junglist i was inspired to put on a manual boost controller set for 10psi. this immediately got my boost gauge up to ~350mm/hg where my popoff valve started singing to me. (see #3 below.) i dislike the MBC behaviour, it gives a cliff-like tail-off for boost, i prefer the asymptotic tail-off i had before my spring crapped out. again, factory spring, 250K miles, 10 years in a field, i'm ready to believe that i had a couple good weeks before deferred metal fatigue set in on my wastegate actuator spring. my son says his electronic boost controller gives better tail-off than the manual boost controller, but i really want to run it stock, which means i need a new wastegate actuator. i'd love to know where to buy a new one that will fit the stock 83 ZXT, so that i can take out the MBC.

 

3. the popoff valve, as mentioned above, was fluttering open at around 350mm/hg on the boost gauge. on the hope that i'm done backfiring into my intake, i replaced the popoff with a 1-inch NPT plug. now i'm getting a solid 400mm/hg on the boost gauge, which means i had more deferred metal fatigue in my popoff valve. i'm not concerned about the popoff like i am the wastegate, but if there's a source for replacement popoff valves i really would like to install one and get it back to "stock config".

 

thanks hugely to all who advised me on this. the problems turned out to be simple like you said, just requiring patience and determination.