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Hi all, I've done a lot of digging via search but have not seen my specific symptoms.
I have just acquired a bone stock '93 FD, and I am getting intermittent boost, sometimes no boost at all, sometimes the boost kicks in late (around 4k rpm but sometimes higher), and sometimes it seems fine. This can happen during the same drive.
I hear a loud hiss sound when there is no boost, but when boost kicks in hiss gets quieter. I am thinking it may be a bad/stuck ABV or CRV, particularly because of the loud hiss that subsides when the boost kicks in?
Has anyone else had an issue like this? I've run through the UIM and what I can get to of the rats nest under the vacuum tank, but all looks in good order.
Troubleshooting the stock sequential system is something reserved for gods and people with a lot of money or time........ or both lol
If you're suspecting the abv and crv you can check by removing one end of their attached hose and looking at the valve position. With the car running the crv should be open and the abv closed. If you rev it somewhat aggressively, you should be able to get the abv to actuate. You can move the throttle by hand while watching it.
Thank you for the suggestion, I'll give it a shot. Yeh I know it will likely be complicated... I also intend to potentially give this (https://www.rx7club.com/3rd-generati...ctive-1120259/) a shot, as the car seems to boost on first pull when warm then sticks. I have a new turbo control solenoid as well that I'll swap in.
While I was fiddling around, the nipple for the vacuum tank broke off. Vac tank looks pretty rough so I'm going to get a new one. I capped off the line and did some testing. Note: This doesn't seemed to have changed any behavior.
No-load revving, I don't see any boost.
For the ABV, it seems to work as intended, there is some air flow that comes out while idling but it is a relatively small amount, a large amount comes out when I rev and suddenly cut the throttle. Should I be concerned about the small amount?
For the CRV, I cannot feel any flow regardless of how I rev it, supposedly it should be outputting air from the second turbo at idle or low revving right?
Finally, I hooked up a cheap boost gauge. Looks like I get 1-2 PSI up to about 3500 rpm, and then 7 psi after that (RPM that it kicks in does vary).
So, my thoughts now are a stuck CRV and maybe Charge Control Valve, directing primary boost into the secondary, and I've added a to-do item to check the restrictor pills (due to the 7 psi). I presume the 7 PSI may also be from having the CRV/CCV stuck still.
the crv is just one part of the sequential system that keeps the secondary boost "off" until the crossover happens. the crv is supposed to be open until crossover and then it closes. if you can put a vacuum on it you should be able to get it to actuate. the 1-2 psi pre crossover could be a number of things but the lowest hanging fruit are a massive boost leak or a blown turbo. the 7psi post crossover adds to the possibility of a blown primary and a little less of a boost leak. there are also a series of doors in the turbos and in the manifold that would contribute to the odd boost pattern as well. checking the actuators to see if the rods are still attached their associated arms will help rule them out..... although not completely.
another thing you can do if you really want to get into it is removing the intakes to get the compressor wheels and checking them for abnormalities. you would be looking for excessive oil, excessive play, their existence lol, damage to the blades themselves and so on. just something other than "normal". in addition to this, you also have a number of solenoids that require verification of operation as well. some in the rats nest and some not. troubleshooting this can get REALLY stupid REALLY fast......
where do you have the boost gauge hooked up? in theory you shouldnt see any boost just from free revving but in reality, you will... very easily.
Thanks for the advice, I added the boost gauge on the UIM nipple next to the one for the ABV.
I edited my post with some additional info (vacuum tank broken). I thought that the secondary turbo wouldn't kick in if the primary was blown (has to get up to pressure)?
I will have to get a vacuum pump to test some of these.
I also find it interesting that the ABV is going off without load but I don't have any boost pressure... I would think it wouldn't?
I've given a quick look and the rods do appear connected, but I'll have to dive a bit deeper.
I think my order of debugging now is
1) Vac tank replacement
2) Check rod connections
3) Silicone hoses & check valves
4) Turbo control actuator solenoid
5) Mess with CCV and CRV to see if stuck
the abv is going to discharge without load. on paper its not supposed to but in reality it will on the twins very easily.
you can bypass the tank with small plastic union for testing purposes. just the connect the hoses that run to it. if you do end up doing a full rework of the rats nest then there are some things to consider. its substantial amount of work to do and the "while youre in there" syndrome sets in SUPER hard. you can find yourself doing way more than just replacing vacuum lines. also keep in mind there is a vacuum chamber that lives under the ps/ac brakcet that requires that bracket to be removed for it be replaced. if the nipple on the upper vacuum chamber broke just from touching it then the odds of the nipple on the lower chamber breaking from trying to remove the line, are high.
very quickly this whole thing can spiral out of control. a domino effect doesnt fully encapsulate what can happen. i always advise people go full non sequential instead doing all this tail chasing. it simplifies the engine bay down to about 5 vacuum lines TOTAL and gets rid of a lot a lot a lot of components. there are advantages and disadvantages to either option and it varies from person to person what those are defined as.
the most important thing about this is where do you draw the line before "gone to far" describes your circumstance? you need to make that decision pretty much now. if the uim comes off and you go into the rats nest, there is no turning back. youre going to have to see that through
Thank you for all the advice, yeh the vacuum tank that had the broken nipple was the one near the PS/AC. The upper vacuum tank is fine, holds pressure.
Yep, fully understood. It's not anything I need on the road at the moment, and I want to spend a solid amount of time trying to get it working in the "as mazda intended" mode. If it goes too long/too futile, I will consider non-seq.
Some progress to report, the passenger side nipple of the charge relief solenoid was disconnected. I reconnected it, and also replaced two check valves, one going from the vacuum tank to the turbo manifold, and one of the other accessible ones under the upper vac tank.
Primary boost has been restored (mostly)! It builds nicely to 8, then peters out as I go above 4k rpm (it's 5 psi-ish around 5k). Still need to debug why maxes at 8 (maybe boost pills)? Could also be that I live at about 4k ft ASL, not sure how much that would impact boost.
Secondary boost seems to be missing... still planning on throwing in the actuator solenoid and inspecting under the UIM.
In reading the fd3snet troubleshooting guide, it says:
You first need to get the Primary Turbocharger working before attempting to fix anything on the Secondary Turbocharger. The Secondary Turbocharger requires the Primary Turbocharger to generate more than 8 psi to operate actuators that control the Secondary Turbocharger.
So maybe it's time to check for boost leaks and pills...
I tested with a substitute vacuum reservoir jerry-rig attached to the hose where the PS/AC one hooked in. Made no difference in behavior, which I guess tracks if I'm reading correctly that it's used to provide a vac source for the turbo actuator solenoid. Also discovered the wastegate and precontrol solenoids had their electrical connectors likely swapped (dot was on wrong one), but no there was no change in behavior.
I checked that the rods for the turbo actuator, precontrol, and wastegate actuator were all attached. Am I supposed to be able to move these with my hand? They really wouldn't budge with the car off.
The troubleshooting continues, but hey at least the boost is predictable now.
Removed UIM, discovered cracked nipples on the pre-control solenoid (green one). Someone hot glued it a long time ago I'm sure...
Replaced check valves
Replaced pre-control and wastegate solenoids
Replaced upper turbo controlled solenoid (which had more hotglue on one of its nipples...)
Replaced miscellaneous suspect hoses, did not tear down into rats nest as the lower area looked in good order
Put everything back together and... second turbo started working, but at weird RPMs it would kick in and it was idling funny (low)... turns out I forgot to plug in the TPS.
Got that plugged back in, and it seems I can get the stock boost pattern some of the time now, but sometimes it caps at 7-8 PSI. May be worth looking into, but I'm a lot happier; it boosts to redline!
09-15-22, 12:14 AM
Intermittent Boost
