spectre6000

iTrader: (1)

I'm going to purchase a car as soon as I can find one I like, and I've always been interested in rotary engines. Knowing that they're a special breed and require special car and feeding that is essentially just a mystery to me, I've always been shy about it. Today I was checking out an E30 BMW, and the guy mentioned that he used to be really into rotaries. What luck! The next car on my list was an RX7. He agreed to come check it out with me and lend me his expertise (which gave me much more confidence in what I was doing). After checking out the car, he raised some concerns that I certainly wouldn't have known were concerning otherwise.

The car is an 86 RX7 NA, and given the manual window winders and such, I assume it's a base model.

Unfortunately, the seller had driven it a short distance before we got to it, and it was slightly warm. This didn't allow me to see it start cold. When it did start (which took a few turns, but nothing excessive), he had to feather the throttle to keep it going. I searched for this symptom here and on the greater internet, but I must lack the proper vocabulary to find someone who has had a similar issue. It eventually got warm enough to not require feathering of the throttle, and began occasionally backfiring (I think) through the exhaust. The rotary guy I brought with me said this is the nature of the beast, but I seriously met the guy just an hour before. No overt reason to doubt him, it just seems like the sort of thing that would have been engineered out before an engine reaches consumers.

The guy I brought with me was keen to smell the oil and the coolant. Interesting approach. I didn't see any oil in the coolant, but there was some foam in the oil cap. The seller claims he doesn't drive it too much, and it sounds like the oil may never fully get up to temp. I'm operating under the assumption that it's just condensation in the oil from the water not getting cooked out from so many short trips in the cold weather. I don't want to make any false assumptions, so I'm leaving this here just in case it's a red flag. I assume my friend was smelling for gas or exhaust in the coolant and oil. Should rotaries have a certain smell to the oil/coolant that I'm unaware of?

Next, the oil pressure seemed pretty high. My guide pointed this out. It was about 60psi on the factory gauge (fairly cold by my reckoning) at idle, and it increased to 100psi or so at full tilt. After it got a bit of heat in it, we were messing with the climate control panel, and noticed it dropped to 50psi (though maybe as low as 30psi at one point) or so at idle. The AC doesn't work but the compressor is still there and belted, so it could have had something to do with load. My guide suggested that I believe is the oil purge valve could be bad and it's not getting vacuum to the crank case. He believes this is the cause of the slightly higher than expected oil pressure.

My guide suggested I get it into second shortly after leaving a light and just flooring it while paying special attention to any dips or hesitations around 3-4K rpm. It was smooth all the way up. I was focusing on control and not getting too illegally fast, so I don't recall what was said or if I pressed on the subject. Something about a numbered throttle valve. Looking into it online, I'm pretty sure I was testing the high rpm throttle bodies (or whatever they are) that open up to let it breathe better at higher RPM. I'm not sure if I should or should not have felt a completely smooth increase in power.

One last major thing was a small rocker switch installed by a previous owner that I'm told operates the fuel pump. He assures me this is a common modification on RX7s, and that he was told they did it on all sorts of cars (though I've never seen or heard of someone doing that). Does that ring any bells? Is this common? Why would someone do that?

There were electrical issues with the wiper switch (looked into, feeling fine with getting that sorted), AC not working (I don't own any other cars with AC, and might just remove it from this one), and the diagnostic stuff doesn't work. Maybe some other things, but nothing I'm super worked up over. There were paint chips here and there that had resulted in light rust, and the passenger side front fender had a good amount of full blown rust behind the wheel, though it seemed isolated to the fender itself.

The transmission felt great, there was a good amount of power, it has a recent emissions test report that shows it passing with flying colors... I think it's a pretty good car. Asking price might be a touch on the high side, but hopefully he's a little flexible there should I not have uncovered anything too dramatic.

j9fd3s

iTrader: (3)

just to kind of go in reverse order...

wiper switch is a common problem with these, fix isn't too bad, if you're not comfortable with soldering, lots of people do it.

the non working ac; the AC system is 100% standard, nothing special at all about it, and at 27 years old, could be a bunch of things, but again its just a normal AC system. in fact most of the car is totally normal, although it does have a side of 80's weirdness

fuel pump switch: these cars flood pretty easily, couple a rich starting map, with any number of things, vacuum leaks, are common, and it'll flood. it really shouldn't need the switch, but people do it.

power: this model of 13B has a pair of rotary valve intake ports, so at low loads they are closed, and high loads they open up. in concept its a little like Vtec, Mazda had it in 1983... if its working right acceleration will just be smooth from idle to redline. if the valves are a little sticky, you might feel a slight flat spot when they open.

oil pressure, the gauge moves. spec is something like 70psi by 3000rpm, and idle will either be like 20-40 depending if its warm or cold (it should be low cold, and go up, there is a thermal valve in the engine for a quicker warm up).

oil cap: you're right, that white goo is just condensation, nothing to worry about at all.

have fun with it!

spectre6000

iTrader: (1)

Thanks for the response. There's still a bit left unanswered or still needing clarification though.

In order of import:

The need to feather the throttle at idle combined with the occasional backfire. I assume the backfire is due to an overly rich mixture, but still shouldn't be happening and the feathering of the throttle doesn't seem right. What could be going on here?

What could be causing the high oil pressure? Is my guide's suggestion of the Oil Purge Valve likely accurate? It doesn't seem to me on the face that manifold vacuum would be sufficient to reduce oil pressure. It may not be necessary to reduce it though... I'm really not sure how high is too high, and only suspect it might be at my friend's suggestion.

I've exchanged a few more texts with the seller seeking clarification on the rocker switch, and it for sure operates the fuel pump. He turns it on before starting, and turns it off after starting. He says it's a common mod. Is it a common mod? Is there some frequent failure of the factory electrical system related to the fuel pump that would cause people to do this en masse?

invinciblejets

I'd be more worried about it having trouble starting warm.
That's low compression I thought.

spectre6000

iTrader: (1)

I want to say we tried to start it again once it was warmed up, and there was no issue whatsoever. It is a common sense thing to do when inspecting a car for purchase (I mostly deal with much much older cars, so I have some odd biases), so I'm sure it was done. Nothing stood out in the hot start arena. It was just cold.

The more I look into it, I wonder if there's something that can go wrong with the AWS system that would cause it to simply not function and the feathering of the throttle was essentially a mandatory surrogate to keep it running until it warmed up. If it was getting extra fuel (which it should have been), that would thoroughly explain the occasional backfires I was hearing. I'm not sure whether or not those parts of the system are directly linked though... The seller had started it up and moved it across a parking lot before we got there (apartment complex), and it was slightly warm. Maybe it was just warm enough to cause the AWS to not actuate, but not quite warm enough to not need it? How does that system work exactly?

What is the max acceptable oil pressure in one of these?

Is there a common fuel pump shutoff mod for any particular reason? Anti-theft device maybe? Is there a common fuel pump electrical Gremlin? Is the fuel pump in the tank? Maybe it failed and it was easier to strap an aftermarket pump outside the tank somewhere and that's how the guy decided to make it run? Are there any bells going off in anyone's heads, or is this just one of those "special" mods unique to this particular vehicle?

clokker

iTrader: (2)

There are a couple of cold start components- the thermosensor and thermowax, specifically, that probably need replacement or adjustment.
If you buy the car, you'd probably be going through all that stuff anyway and I wouldn't worry too much.
If you're reading from a stock gauge, where are the numbers coming from?
You could get a real gauge and know for sure but hell, low pressure would be more of a concern than high.
And no, the purge valve has nothing to do with oil pressure.

The "fuel cut" switch is a bandaid, applied by those too lazy/poor to actually fix the problem.

If you have leaky injectors, after the engine stops residual pressure dribbles fuel into the "cylinder", flooding the engine. If you install the switch, you can kill the pump as the engine runs, empty the fuel lines before shutdown and avoid the problem.

Or you could have the injectors serviced and avoid the mickey mouse ****.

spectre6000

iTrader: (1)

That's the sort of thing I was looking for! Awesome. I'll be doing quite a bit of work on it, no doubt, I just want to make sure I'm not setting myself up for failure. I'm going back out again tomorrow (assuming I can coordinate it with the seller), and I have a few more things to look at before I'm ready to make an offer. Might have a new (to me) RX7 in the drive tomorrow night!

clokker

iTrader: (2)

Where in CO are you located?

spectre6000

iTrader: (1)

Broomfield (mid-way between Denver and Boulder on 36).

j9fd3s

iTrader: (3)

these cars have a lot of stuff for cold starts. there is a thing called the thermowax that literally pushes the throttle open when its cold, this gives it a ~1600rpm cold idle that slowly drops back down to the spec of 750rpm +/-50. there is also the AWS valve, for the US market, they use it to get the catalytic converter to light off faster, this gives you ~3000rpm for 30 seconds, when the coolant temp is in a certain range.

the AWS seems to always work, but the thermowax can fail, and more normally the coolant lines are plugged, and then the throttle position sensor, idle speed and mixture are all adjustable too, so there are a few things that need to be checked before anything gets replaced.

it a fast idle it will misfire/backfire a little (maybe less than 10 a minute?), but at 750rpm it should idle smoothly.

spec is 71psi. a stock 86 Rx7 won't go over that. however all the racing engines and the later FD were set to 110psi. so between the flaky senders and possible mods, max acceptable oil pressure is whatever its doing. you don't need more than 71psi, but you can't have too much either.

if we haven't posted them yet, here is a link to all the manuals we can find that i'm not too lazy to scan Foxed.ca - Mazda RX-7 Manuals i'd suggest starting with the training manual, as it explains every system on the car quite well

spectre6000

iTrader: (1)

At what RPM?

I went on a manual hunt last night, and downloaded everything I could find (including manuals from that site). FYI to any mods bored enough to be reading this, several of the manual links in the FAQ are dead.

What I haven't found yet is an owners manual. The sort of thing that shows where all of the switches are, what they do, expected owner maintenance and scheduling, etc. Are those available anywhere for download?

clokker

iTrader: (2)

Well, if you'd like another pair of eyes on your purchase, perhaps we can work something out.

spectre6000

iTrader: (1)

I lucked into a far more educated set of eyes my first time out, and I'm pretty thoroughly solid with piston engines and engines in general, so I'm feeling pretty good. I'm meeting the guy downtown in about 45 minutes to see it start stone cold, check the undercarriage for rust and such, see what I can figure out about that switch, and potentially make an offer. If you feel like you really want to meet me downtown at 1:15, PM me your phone number and I'll give you the details. Either way, I really appreciate the offer. Maybe once I'm being pulled around by spinning triangles, you could introduce me to the clubs, other enthusiasts, what have you in the area so that I might be fully immersed in all that is the Mazda rotary experience.

clokker

iTrader: (2)

I'm not much of a scenester and have met but a few far-flung owners.
I can't recall the last time I saw another RX (FB/FC/FD) on the street.

j9fd3s

iTrader: (3)

i think around 3,000rpm? there is a thermal pellet in the eccentric shaft which cuts the oil cooling to the rotors when the engine is cold (under 65c oil temp), and it does this by dropping the oil pressure. so if everything is working properly a little after coolant hits 65c, which is the first line on the gauge, you will see oil pressure go up ~10-15psi when the valve closes.

good point on the owners manual, i guess the service manual has all that stuff in it, but not in a user friendly way. its a good thing the car is laid out pretty well, and there are places like the cold start system where there is a lot of stuff, but each piece on its own is very simple.

spectre6000

iTrader: (1)

I want to say at or just above idle, it was about 70. Probably a bit higher at 3k though. What could cause it to be so high? It could have been a little cold, FTR. I'm pretty sure the AWS is non-functional. I just got back from my last little check/offer, and I made sure the engine was stone cold. It started up pretty quick, but immediately only idled at about 1K or just under. If my understanding of the AWS is correct, the thermal wax sensor is dead, but the good news is that it likely has fantastic compression.

j9fd3s

iTrader: (3)

its supposed to be that high. the race engines, and the later FD's are set to 110psi. the more the merrier.

the cold idle thermowax isn't a sensor, it is a wax pellet that literally pushes the throttle open when the engine is cold, this gives a ~1600rpm fast idle cold. the AWS gives a ~3000rpm for 30 seconds fast idle, to light the cat off, and it is only done in a certain temp range (its in the training book).

lduley

iTrader: (1)

But, if its started in gear it will not shoot up to 3K IIRC

spectre6000

iTrader: (1)

"Thermowax" = wax thermostatic element = theromstatic sensor (I used the most generic word since I'm not yet hip to the lingo). I understand that I need to learn the terminology though. The internet is a difficult enough communication medium even with a shared lexicon.

It was started in neutral with the clutch fully depressed. It turned over for about a second, then caught and idled at ~1K RPM. No 3K, no 1.6K, just barely 1. Maybe a touch faster than it would idle warm, but if I understand correctly, the "Thermowax™" thingy is responsible for the physical actuation of the throttle to get the idle up. I haven't had a chance yet to verify this completely, but if that is indeed the case, It seems like it's likely pretty thoroughly dead. It's 30° here right now in CO, so I'm pretty sure it would have to have been well below whatever threshold Mazda's engineers would have seen fit to begin upping the idle.

The race engines may idle with a very high oil pressure, but do they have any sort of modifications to make this a safe constant? Is there a point at which some line or gasket in a stock (tired) NA 13B will let go? I know some engines are designed to make high oil pressure to support stressed bearings, and it may be this is exactly what's going on here, I just want to make sure before cash changes hands. An offer has been made, but until the deed is done, it's not too late.

clokker

iTrader: (2)

There are two bypass/relief valves in the system. IIRC, at 100psi they dump oil back into the pan, saving the system.
The most common failure point in the oil circuit is the lines from the block to the cooler.

As for the thermowax...
It ends with a plunger that rides on a cam in the linkage. It is literally filled with wax and gets water from the back of the waterpump housing. As your coolant heats up, so does the wax and as it melts, the plunger retracts. It's common for the water passage to get blocked, which screws up the process.
The plunger is also adjustable, it may take just a turn of the screw to get back in range.

It's a bonkers system but makes sense when you see it.
Which you will.

Unless you have evidence to the contrary, one of your first "general maintenance" moves should be to replace all the vacuum lines and water hoses, which is a fairly massive (but not really expensive) job...the entire intake must come off, all thirty some pounds of it (35lbs., actually).
The throttle body will come off right at the beginning and you'll see the thermowax.
This is what the intake looks like off the car:


The vacuum spider is below the intake, sandwiched between the bottom of the manifold and the top of the keg.

Oh, what fun you'll have.

j9fd3s

iTrader: (3)

lol, too true, just clarifying it is just a dumb wax pellet not connected to anything but coolant.

its common for the coolant hoses to the thermowax to be plugged, but the pellets sometimes fail too,

nope! the later street engines had the higher oil pressure too.

spectre6000

iTrader: (1)

Cool deal. I'm feeling very good about the car. It has some issues, but I've looked at too many cars now, and it's the best by far. It sounds like the deal is going down tomorrow evening.

First thing it needs is some brake work (actually, probably a good wash). Feels like the PSF rotor needs a new surface and pads to match. Probably going to tackle the whole mess Sunday or maybe get started Saturday evening. Might even do the brakes sooner if I can find the time and some decent weather. My truck is occupying one half of my shop space, and my restoration project has the other half right now, so I'm essentially stuck outside. Oil was recently done, but I'll do that anyway just to get a good baseline, transmission oil, differential oil, coolant flush, brake fluid flush and probably convert to DOT5 (I think I still have a half gallon or so from my truck), and I'll check out the various hoses and lines for pliability and replace anything that looks remotely suspect. That's probably all I'll be able to do that first day (assuming I can even get that far).

Next, I'll have to track down an electrical gremlin or two... See if I can't get the diagnostic system to work, wiper switch, and I'll tear down, clean, and regrease the sunroof mechanism. My best friend is a research physicist at CU and a bit of an electronics guru. He gets giddy when I talk about the sorts of projects I'm likely facing with the computer stuff on this thing, so I'll bet that ends up being fun.

Any recommendations for various fluids? Is there some special oil that is the community's favorite, or is that a hot button issue?

Clokker, do you feel like showing a rotary noob the ropes? A practical tour of the engine bay would probably go a long way, and I'll bet there's beer in it for you. Maybe even an experimental homebrew stout (yeast experiment) if I have any left.

clokker

iTrader: (2)

Sure, love to.
I don't drink but Coke will do nicely.
PM me and we'll set something up.

GrossPolluter

iTrader: (1)

If I were to buy a car the main thing I would look at is accident damage. All the mechanical problems can be fixed

spectre6000

iTrader: (1)

Generally, I'd agree. Actually, I'm typically OK with dents and dings. I'm pretty good with a body hammer. What I can't stand is rust... I. Hate. Rust. It has more to do with the complete lack of replacement body panels on 50+ year old cars, and is probably a bit irrational for something like this, but that doesn't alter the sentiment. Regardless, it's my new daily driver, so I'm a bit more concerned with the mechanical stuff at the moment. Once it's flawless mechanically, I'll worry about aesthetics and creature comforts.

My winter daily cracked its block, so it's down (fortunately, I work from home, so it's pretty easy for my wife and I to just share her car). My summer daily is far too nice for winter roads and soccer moms, and it's worth a lot of money. My restoration project is still in bare metal and boxes without a drivetrain at all, so it's pretty much not an option. Finally, we had a pretty incredible flood here a year and a half ago, and we got hit pretty hard. So we're currently living in an apartment that has a 4 vehicle limit and very minimal shop space... So I sold the summer daily to get a new engine for the winter daily (it was too nice for me anyway... I prefer beaters), pay off some debts, and pick up a cheapo little summer daily. I initially set out to get a cheaper cheapo little summer daily, but it's hard not to talk yourself up to something nicer sometimes. The immediate goal is daily driver while I get the truck back on the road, then it'll be a fair weather car for running the canyon roads up where we (used to/will soon) live and maybe see occasional track use once it becomes a bit less mission critical.

Hopefully I'll find the time to figure out what the norms and mores are with this group sometime this week (I spent Friday through Monday hunting, and need to catch up on work), and I'll get a build thread (or something to that effect) going once it's in hand.