That's certainly what it looks like, but the EGR is in under seat engine compartment.Originally Posted by TheMAN
It may be a moot point though, since there is a problem with the vacuum line. I've replaced it twice (not with OEM hose) and finally got so disgusted I just plugged it off.
I replaced the bad vacuum line with a hard plastic type hose from Autozone. I had to heat the ends a bit to get a nice tight fit, but the vacuum leak is now gone. I also had a broken PCV grommet and replaced that too. Note to self: anytime you remove the PCV valve, replace the grommet so you don't have to remove the valve cover to extract the broken pieces!
I bought an OBD scan tool and the Haynes ODB-II Techbook and with the vacuum leak, the Short Term Fuel Trim was way off >13, but after replacing the vacuum line it was oscillating around zero +-2. After reading through the book (which I wouldn't buy again), it seems like I've done all the applicable things and replaced all the likely suspects, but one thing I read that I hadn't considered is exhaust back pressure.
Since the problem of shuddering/backfire only occurs at high revs (>2500) I'm wondering if this might be the cause. Has anyone else run into a similar problem or is this just something they added to fill up the manual?
I'm not saying it's back pressure, but that could explain things. About the only thing that can create a back pressure issue is a plugged up cat converter........and yes, this can create massive performance issues. The few times I've seen this it was also accompanied by a melted EGR modulator (too much heat due to back-pressure). It's possible yours might be starting to plug. The only way I know to test is to run it with a temporary test pipe or take a very loud and obnoxious test drive with an open header. I've been following this thread, but sadly I don't have much to contribute as I've never owned or worked on a supercharged Previa. Good luck. Tim
The Hayne's manual says the test for back pressure is:I'm not saying it's back pressure, but that could explain things. About the only thing that can create a back pressure issue is a plugged up cat converter........and yes, this can create massive performance issues. The few times I've seen this it was also accompanied by a melted EGR modulator (too much heat due to back-pressure). It's possible yours might be starting to plug. The only way I know to test is to run it with a temporary test pipe or take a very loud and obnoxious test drive with an open header
Test the vacuum pressure (at the intake manifold) at idle and if the pressure slowly drops toward zero, then there is a restriction.
It goes on to describe the test for an exhaust restriction as:
Record the idle pressure, then rev to 2000 RPM which should reach about 16 in-Hg and then quickly release the throttle. The pressure should lower at the same rate that it increased. If the vacuum pressure is 5 in-Hg or more higher then there is an exhaust restriction.
Course, once having found there is a restriction the only real test is to remove the exhaust manifold.
I'm wondering if simply removing the pre-cat O2 sensor would provide enough of an air leak to test it for blockage to avoid taking off the header?
I'm on vacation in SC and the area is generally agricultural, but I'm loathe to pull off the exhaust header if I can avoid it....cuz the SC police just love to ticket the vacationing New Englanders.
No reason to pull the header, just pull the cat. Quite often a visual inspection is good enough to verify. Just shine a bright light in one end, and look through the other end. You should see light through the mesh inside. If it's pitch black, or if there's junk in there (like balls of soot), then you found your problem. If that's the case, you can gut it out by ramming a rod through it and break out the guts. After that you can bolt it back up and use it as a test pipe. Test drives with open exhaust are always a fun way to verify, but usually are not necessary. I've never done the visual inspection with a Pevia cat before, but think it should work. The only problem I can think of is all the pipe that's included as part of it. This might make it harder to inspect. If the pipe makes it visually difficult, you could always use an inspection camera (assuming you have access to one). Tim
Gee Tim, you make it sound so easy.
From the looks of it, the exhaust system is original to the van and since I'm on vacation I don't have a spare vehicle to fetch replacement hardware for the case where the bolts are [invariably] frozen. So, I'm probably in for a penny, in for a pound once I start to disassemble. I do have an inspection camera, but no rebar (or other suitable destruction tool). Regardless, it's a slippery slope under the circumstances. I'm gonna test it with the O2 sensor removed. If the hesitation problem goes away, at least I'll know that backpressure is the problem, and if it doesn't make any difference then I'm back to square one anyway.
if the engine ever ran rich before, it would've melted the cat, after quite some time
gutting the cat will result in a check engine light due to OBD2 monitoring for cat efficiency... the light should have came on by now if the cat was bad, but then again, we're dealing with early OBD2, it might not be so smart....
Update: I took off the pre-cat O2 sensor and it seemed to make an appreciable difference! There was still some backfire at higher revs but I couldn't say whether that is a function of my hack, or if removing the O2 sensor didn't release enough pressure, or if the problem persists. These vans seem to have a lot of variables with the air/fuel system, so it wouldn't surprise me if the lack of the O2 sensor was a new culprit. I didn't put a vacuum gauge on it, so I don't know the exact difference the increased air flow made with respect to vacuum pressure. I also did not take it on an extensive test drive. Regardless, there was enough improvement for me to justify replacing the cat.
The engine had been running rich for some time with various engine codes indicating bad O2 sensors. Thing is, I don't really know when this stuttering/hesitation problem started and I've made so many repairs lately that it's hard to remember the order of things but I had replaced the O2 sensors...twice!
Course, hindsight is always 20/20, but as I reflect on it, excess backpressure in the exhaust may have increased the heat retention in the engine which may have been a component of the head gasket failure. Likewise, it may have had something to do with the vacuum system problems that manifested themselves in many places: EGR, Throttle body, EVAP, all of which were repaired at one time or another.
All the maintenance stuff that was done may have compensated for the problem with the backpressure which may actually be the root cause...and the supercharger adds some additional complexity to the air flow dynamic and is completely foreign to me.
Update: I finally got the new cat, but it was the wrong part....but of course, I had already removed the old one before realizing it. Regardless, while I had the old cat off, I took a peek inside and could see light shining through. Then I used my handy dandy scope camera and verified that it was in good shape.
Since back pressure isn't an issue, I'm starting to think that I must have F'd something up when I replaced the head gasket. Course, the van is running excellent at low speed (<60MPH), so whatever I did wrong won't be obvious.
That said, while I had the head off, I lapped the valves and replaced the valve seals. What I didn't do is reshim the valves (though I was very careful to put back everything in the same place from whence they came).
Is reshimming the valves a critical component of a head gasket change? Does anyone have any instructions on how to go about the process and whether this may be the problem?
Thanks
Probably not the valve shims. I doubt you could have removed enough material with a hand lapping to create an issue, but not a bad idea to check it out during a major service like that. Due to wear on the camshaft lobes, shims, and valve stems, valves can get a bit loose. A slightly loose valve (due to normal wear) is no big deal. Wear on the valve head & valve seat can however make the valves tighter. If you only hand lapped to verify a good seal, then you're probably okay. If however you worked it hard and removed a lot of material, this might have created an issue (but not too likely). Tight valves can be a problem because the valve relies on it's contact with the seat to cool (especially true with exhaust valves). For this reason, tight valves typically run hot & valves that run hot have a tendency to burn or warp. I doubt you have this issue because burned or warped valves do not seal and there would be an obvious miss (especially at idle). Since intake valves are cooled by air/fuel mix coming through them they usually don't burn or warp (this is also the reason intake valves are typically adjusted tighter than exhaust valves). Severe intake valve leaks are usually accompanied by backfire noises up through the intake. Severe exhaust valve leaks are usually accompanied by backfire noises out through the exhaust. Severe valve leaks (of either kind) can also be detected by a compression test and/or by listening to the cranking of the engine with the ignition disabled. If you have a compression problem in a cylinder that is not improved when oil is introduced, then that indicates a bad valve.
If the manual you are using is anything other than a genuine Toyota factory service manual, then I'd highly recommend you get the Toyota one. There is a highly detailed area that explains setting valve clearance. The FSM is very thorough (perhaps to a fault) in some areas. Here are links to a couple of threads where I attempt to explain setting valve lash. http://www.toyotavantech.com/forum/s...Interchangable, http://www.toyotavantech.com/forum/s...ia-head-gasket. The 1st thread is short. On the 2nd one this is discussed near the bottom of the 1st page. I'm guessing this is not your issue, but after the head has been off it's always a good idea to check & adjust as required. Tim
Thanks Tim!
I did lap the valves by hand, but the van does backfire at higher RPMs (>2500). I also had sent out the head to get resurfaced. The cylinders all had pretty good compression (~120PSI) prior to the head gasket change and I had read your posts which were the impetus for testing for valve leaks and subsequently lapping the valves.
I don't have the Factory Service Manual, nor do I have the special Valve Clearance Adjustment tool. I did find a Youtube video of this procedure. Regardless, I appreciate the info and it's good to know that the likelihood that I screwed things up significantly is minimal! I've ordered a micrometer that should arrive tomorrow.
During the process of reinstalling the old catalytic converter (after receiving the wrong part), I replaced the gaskets on both ends. I went for a long drive yesterday and the van was running noticeably better. I'm not sure if the gaskets tightened things up with respect to air flow or whether it was just coincidence, but as long as I didn't abuse the throttle, the van performed very well through the driving range up to around 70MPH. It eventually started to sputter around 72-75MPH, but it was a very pleasurable ride!
Last edited by jmrodz0; 02-19-2015 at 11:07 AM. Reason: misinformation
If the machinist ground the valves, I would be concerned about adjustment. A light hand lapping only would not likely have much affect on valve lash (adjustment). Replacing gaskets on the cat would not have an effect on performance. Might however affect sound, which can change your perception of performance. Tim
Thanks Tim! Sound like I couldn't have screwed things up too badly by lapping the valves so that's great news!
I haven't adjusted the valves yet. I'm heading back to MA in 5 days so I don't want to get too creative at the moment. The van is running well at low RPM so as long as it gets me home, I'll revisit when I get back to the frozen tundra.
Since I will be checking the valve clearance, I'm thinking that I will verify the camshaft installation at the same time. When I originally installed the camshafts, I had set the engine to TDC and tied the timing chain to the sprocket, but I didn't pay any attention to the actual timing mark on the crankshaft pulley.
After reading through Tim's excellent write up on rebuilding the 4yec engine, I notice that he set TDC with a gauge on the piston and then redefined a new crank pulley mark to align with the "0". My question is whether this same procedure applies to the 2ZFZE engine as well?
My plan now is to set the crank pulley mark to "0", then check the camshaft orientation and reinstall them if the timing marks on the cams are not properly aligned. However, if the crankshaft mark does not take precedence over the cam position and sprocket, then I would align the cam timing marks and redefine the crankshaft pulley mark to correspond.
Are you saying that you never checked the ignition timing after doing a head gasket? Ignition timing is important and needs to be set any time the distributor is disturbed (as required while doing a head gasket). I wouldn't worry too much about the accuracy of the timing mark on the crankshaft pulley. Besides, with the head installed, there's no easy way to check it anyhow. If you're concerned it's off, you could always get another one from a salvage yard and compare (the odds of two being off the exact amount are almost nil). FYI, these are almost always accurate.
The pulley timing mark being off that far (in my overhaul thread) was an exception to the norm (unusual). When checking the ignition timing on Previa, you'll need to jump the pins in the box under the driver's seat to put it into the diagnostic mode (refer to that section of the service manual). With a timing light, check the orientation of that pulley notch with the marks on the timing cover. Adjust the timing only while in the diagnostic mode, then pull the jumper and check with the light again to verify the electronic advance is working (timing should be different with the jumper removed). Good luck. Tim
Thanks for the information Tim. That's what I was hoping to hear.
FWIW, I had marked the distributor orientation during the head gasket repair and when completed I took the van to the local dealer to set the timing. However, after they probed around for 45 minutes, they informed me that the timing on this model year cannot be manually adjusted! I have tried resetting the electronic timing with a jumper across the TE1/E1 terminals under the driver's seat, but as far as I can tell, there is no other way to adjust the timing.
I'll admit I know nothing about SC models, but I find it a bit hard to swallow there would be no ignition timing adjustment. On NA models the distributor set bolts are in slots. To adjust ignition timing you loosen these bolts, then rotate the distributor to adjust. Once correct ignition timing is established, you tighten the distributor bolts to lock into place. After tightening the bolts, I will usually double check with a timing light (to make sure tightening didn't move things). If you have the 94 factory service manual that should shed some light. I'd look it up for you, but my newest manual is 93, and there were no SC models until 94. Tim
Thanks Tim. This '94 LE/SC model does not have slotted holes in the distributor, just round ones....and I was just as incredulous as you when I was told of this. Course, I'm a bit of a dope for not realizing this when I had the distributor out, but since I had marked the distributor location before removing it, I didn't even think about future adjustments.
Do you have the 94 factory service manual? If so, look it up. If not, you should focus on getting one, then look it up. Tim
I don't have the Factory Service manual, but I'll check out ebay and see what they've got.
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