So there is a bit of a story to this before I get into the full technical details, but any help with this engine would be greatly appreciated...

Here we go. I bought a 1990 rx-7 convertible from a friend of mine about a year ago. 159,000 on the odometer with a rebuild 40,000 miles ago. The information he left me with was foggy at best but I will try to describe everything he told me. Now I knew about rotary's before he sold it to me but not much. The car started and ran fine nothing was majorly wrong with the car except a minor exhaust leak. He had changed out the exhaust manifold to an OBX Racing header exhaust. He also explained that he had a bridge port job done to the engine at the time of rebuild(all work done by Mazdatrix) and a few minor modifications such as a cone filter to bypass the stock intake and new wires(all that I remember off the top of my head).

Now unfortunately I had to let the car sit for about 6 months while I went out of town. When I got back the car had the normal symptoms of having stale fuel, so I cleared it out and changed the gas/oil/coolant. However a week later the idle seemed extremely erratic, surging from 750-250 and sometimes dying out when cold or dipping so low the starting lights come on as if it is going to die only to turn back on. Driving seemed fine, no major issues there. One day I notice the sound of the car is off and it sounds waaaaay too loud. I take a peak under the car to find the exhaust leak has turned into a full blown split in the header with a nice half inch gap between pipes. After inspection I find that the cat was plugged up which probably caused enough heat and back pressure to blow the headers. I've driven it a bit like this(I know I shouldn't have) but now new problems are showing up. Hesitation while accelerating through gears(usually only 1-2, once it gets over 4k RPMs it's fine), sometimes the car cuts out even at high RPMs as if it isn't igniting then turns back on when the clutch is depressed, and today I had the final problem... The car wouldn't rev between 1-3500 RPMs and even after that it was really shotty. When I was able to drive off, it was under 30 mph and sounding like it was missing still. After parking the car for a few minutes and running it again, the problems persisted for about 600 ft and then it magically disappeared and the car ran fine again.

I know I haven't treated this car with the love and respect it deserves but I'm in a place now where I have the ability to get things fixed and I am just looking for a bit of help as to what the problems may be and where I should start. Any assistance is greatly appreciated.

-Jon

TL;DR
Idle surging from 750-250 when warm
Idle dying when cold at times
Backfiring
Split exhaust headers
Hesitation while accelerating
Ignition disappears at high RPMs occasionally
Finally almost died on me with erratic ignition at 1-3500 RPMs and sputtering at 4k+, wouldn't go over 30 MPH, etc.

 

I realized todayi forgot to mention it's an n/a motor

 

If no changes were made to a clogged exhaust system then things would probably only get worse over time so this is probably a good area to address. Fix the obvious and then see how things are.

 

The cat was gutted but the pipe is also split pre-cat anyways so exhaust is coming out from there. No more clogged exhaust so I know that's not an issue.

 

With a gutted cat and a split header, it's possible that you don't have enough exhaust back pressure to operate the auxilliary ports. This usually happens about 3800 rpm & may explain your hesitation while accelerating.

 

doesn't apply to the s5 Rob..S5's operate off of vacuum solenoids.(VDI)

 

Thanks for the correction...

 

So now we've got a few things it's not, what could it actually be? I'm about going crazy over this thing.

 

have you attempted to pull any codes yet?

 

I have yet to pull codes on the car, I will certainly try that tonight. But other than pulling the codes is there any help that can be given regarding these problems? Haven't driven her in almost a week due to fear :s

 

Probably ethenol clogged your injectors. Happens on my boat all the time when i let it sit.

 

Or it may have burnt some wires when the header split?

 

Okay so I finally pulled the codes and I have a bit of confusion but maybe someone can help clear that up here. The codes I kept getting were either a 10 and 3 or 13... Kinda difficult to tell if the pause was long enough. Codes being

03 - Crank Angle Sensor (G) - Fuel Injection and ignition cut
10 - Intake Air Thermosensor (AFM) - Intake air fixed at 68F
or
13 - Pressure Sensor (intake manifold pressure) - Fixed at 29.9 inHg

So with those 3 codes and my initial symptoms, is there any further assistance that can be given? How can I go about fixing these problems?

 

anyone?

 

Disconnect the plug at the CAS. Then take a multimeter set to ohms and place one meter lead on the White wire and then place the other meter lead to the Red wire from the Cas pigtail harness (Yes, the one that is short and comes from the CAS itself) and take the ohm reading. It should fall between 110 to 210 ohms. While you're there also take an ohm reading from the Blue wire and the Red wire. Should fall within the same range.

 

On the second reading, use the Green and Blue wires and not Blue and Red. Sorry about that.

 

Went ahead and took the readings, red/white is 170, blue/green is 170. Now I also noticed that the attaching connect is backwards... Not sure if this will make sense but the little clip part is matched up correctly but the plug from the CAS and harness look like this
GB RW
WR BG

So they are "reversed" connected... Wires don't match up. But like I said the clips match up perfectly... Any help here? Also, thanks for the help.

 

When you mate the two plugs make sure the connection is good and clean as well. Now take the same measurements at the ECU as you already know via this test that the CAS itself falls within the specified range. Pin 3G is White, 3H is Red, 3E is Blue. There are only three ECU pins related to the Cas even though there are four Cas wires, and the reason is the Red and Green CAS wires eventually mate together before reaching the ECU. So at the ECU remove the plug housing these wires and take the ohm reading for the White and Red wires and a reading for the Blue and Red wires. And if these readings are good then make sure all of these wires are firmly pushed into the ECU plug housing these wires.

 

wow gotta say that was one of the hardest things I've done in awhile. Had to find the ecu in the car, then find a wiring diagram, and well that was alot harder than expected. Anyways! readings at the ecu were good aswell. Sitting at a solid 167. I disconnected, cleaned, and reconnected the plugs on both ends. What's next?

 

Are you sure you're getting the right codes? You can pull codes at anytime by grounding the Orange wire found in a Green check connector right near the Leading coil. This connector only has the one Orange wire. Key to on after the wire is grounded and the codes will show.

 

Yeah I did that. Grounded the connector to the negative terminal on the battery, went inside, turned key to ignition, and watched the codes. It was 1 long light(maybe 1 second) followed by a short pause (maybe 2 seconds?) then 3 quick flashes(half send a piece) followed by a long pause(maybe 5 seconds) then it restarted. Like I said, it was either a 10 and 3 or 13. Kinda difficult to decipher.

 

Well the CAS checked out ohm wise at the ECU so maybe it's not a error code for the CAS but another item. If it's code 13 then just disconnect the Pressure Sensor and check codes again. If you had two codes before and they were a 10 and a 3 then you would get a 13 w/the sensor disconnected or three codes total. If you get one code though then you know it was a 13 and only a thirteen..

 

Other codes involve the Intake Air Thermosensor(AFM) and the Pressure Sensor(intake manifold pressure). I've check the connectors and vacuum lines on the MAP haven't yet checked the Intake Air Thermosensor however.

I'll see what I can do about maybe getting a video posted of what the car sounds like and looks like so I can get a bit more diagnostic help, but as far as all the other problems go is there anything else I can do to check?
Btw, I really appreciate you helping me out as much as you have.

 

Disconnect the plug to the Pressure Sensor and check for codes as suggested in my last post. If you truly had a 3 and a 10 then disconnecting the sensor plug will give you three codes as you would have a 3, 10 and a 13. If you only have a 13 then that's one code but if you see two codes of 3 and 10 then it is likely just a 13 since the CEL should throw a 13 if its' plug was disconnected.

 

Alright I went ahead and did what ya said and it seems as though the code was definitely a 13, which turns out to be the Pressure Sensor(Intake Manifold Pressure) which I believe is the MAP sensor? What can I do about fixing this particular part or is it a simple replace?