ThatWiensFellow

Hello! this is my first time posting on here so bear with me. i purchased an NA 1990 Convertible with an automatic over a year ago and i am still struggling to get it road worthy. my plan for the car is to keep it stock and to keep it automatic.

so here's the issue. the car will start but it will not drive. it doesn't handle any sort of load well. turning on the headlights or using the power windows will bog the motor down significantly. the car will also not idle in gear. the moment it see load it bogs down and dies. it runs rich. both leading plugs are black. it misfires and backfires on decel alot. i know very little about the cars history. i was told when i bought it that it had a reman engine but i cannot confirm that. i do know that the previous owner had attempted to "chip" the ECU and the car would not run with said ECU. because of said ECU the Throttle body has been tampered with. from what i have found the primary and secondary throttle blade stops have been moved and the cold start adjustment has also been tampered with.

Onto what ive done so far. ive been using the S5 factory service manual as my guide. I have fully rebuilt the entire intake manifold. new gaskets, new vacuum lines new check valves. i have checked the brake booster for leaks and found none. i have checked the intake system for leaks and found none. i have reset the TB to the best of my ability, i have checked the BAC, the AWS, the AFM, the Pressure sensor, The main relay, the TPS, both coils (didn't have the tools to check the ignitors), The CAS, the air intake temp sensor, the water thermo sensor, and replaced the OMP with a working unit. all resistances check out and all units have continuity to the ECU. ive replaced the ECU with an un-chipped N353. I've added and tested my grounds. I've done a compression test. Front rotor is 82,87,87. rear rotor is 81,87,80.

this is the part where i need some advice from an expert. i am aware that those compression numbers are low. and the poor compression explains alot. however i have been told by a few people, as well as owning a dead rotary in the past, that cars with compression in that range are still drivable. and i have taken the car around the block once. you have to drive it with 2 feet like a forklift. im not a fan of this solution so kept digging. i found that my ECU has a blow resistor #824. i traced this back to the ECU pin 2N, which is an output to the port air solenoid (emissions). i tried to source another N353 (S5 vert) but could only get a N351 (S5 coupe) which would not run the car. it would start, idle for 3 seconds and shut off. the car starts and runs fine with the questionable N353.

my question is this, is my issue due solely to bad compression and the fact that it is an automatic? Or is my questionable ECU causing my issue? Or have i overlooked something entirely? any help would be greatly appreciated. id really like to cruise some beach roads with the top down.

Thanks.

WondrousBread

I'm an S4 guy, so someone else will have to comment on the ECU.

Your compression numbers are on the low side, but they shouldn't cause that dramatic a problem. You can drive the car around at reasonable speed with a blown rotor. There is definitely something else afoot.

j9fd3s

iTrader: (3)

i'm not 100% sure, but i feel like the N351 should at least start and run the engine. do any of the ecu's give you error codes?

ThatWiensFellow

The N353 ECU is giving me a #33 code. This is for the port air solenoid, which i assume is due to the broken resistor. The N351 ECU i tried did not register any input codes. I didnt check for output codes.

j9fd3s

iTrader: (3)

that does seem logical.

have you checked the fuel pump? fuel pressure? if the ECU is happy, then its a mechanical thing. you compression is even enough, it should run fine

ThatWiensFellow

The car has had every part of the fuel system replaced. Alot of people have said the compression would be fine to drive. However most people own manual cars. On a manual car where you are constantly giving throttle input when load is present I imagine it isnt an issue. In an automatic you are loading the car up at idle. Would that suggest that an auto would fail with higher compression than a manual?

ATC529R

Have you checked it for vacuum leaks. Would seem to be the simplest thing to check.

like with carb cleaner or propane.

and if you have the old parts…as you have changed so many. I would swap in the old stuff one at a time and see if maybe you got a bad new part.

Brrraaap

Could it be a clogged cat? Obviously very different but I had a truck do the same thing with a clogged cat though not as severe. I would start by replacing the resistor and the solenoid just because you know its broken.

ThatWiensFellow

Without divulging too much, i am 100% sure the cat is not clogged 😉. Im in the process of finding another ECU.

j9fd3s

iTrader: (3)

well this one time we had an Rx8 come in running on 1 rotor. we compression test it, and it does 5's on the front rotor and whatever is good on the rear. so engine is toast right? we call Mazda and they have us throw an ignition system on it (plugs, coils, wires), kind of weird, but its warranty, so its their dime. we put all that on the car, fire it up, and it starts running on both rotors, and after it runs for a while we re test it and compression is good. it went from 5's to 8's or whatever just by running properly for a while.

story number 2, there was some piston car with a bad cylinder and for some reason all the techs tested it with their testers. they all got different readings. same engine, same day.

so the actual value of your compression isn't the big deal, and we might expect it to go up after the engine is happy. the thing is that its even, which tells us that the engine is ok

ThatWiensFellow

The car could definitely use some new plugs. Ive checked the resistance on the plug wires and they seem to be fine. I checked the coils them selves but not the ignotors, so there could be a potential fault there. I can see about swapping in another set. The CAS is set correctly and the resistances are in spec, and all wires back to the ECU have continuity. Any suggestions. Is there something i missed? Could checking continuity’s and resistances not be enough maybe? Any suggestions on where to go from here?

ThatWiensFellow

Another point to mention is the manifold vacuum below 1000/1200rpm is very inconsistent. The needle on my gauge bounces from 5-20 in/hg

Brrraaap

If the solenoid is shorted it’ll burn out the ecu resistor again, stuck injector maybe?

ThatWiensFellow

i have tons of emissions stuff to pick from so i will make sure i have a good solenoid before i put a working ECU back in. I am of the belief that it was popped when i got it though. As for injectors, i was a fool and didnt have them cleaned when i had it apart the first time. I did check the resistances and they were a heck of a lot cleaner looking than the last set i put in my other car. Is there a way to tell if one is stuck without taking the whole manifold off?

Brrraaap

I had the opposite problem, mine weren’t firing, if you turn the key on and pull the CAS, push in the AFM, and spin the CAS it you should hear injectors clicking, something I did was to spin it a few times and then crank with no plugs in to see if you get the same amount of fuel.

Are you sure that there’s no leaks between the afm and the TB because that hose can leak and if it leaks it’s a big leak.

ThatWiensFellow

So that’s a tricky one. There are no big leaks. All the hoses are fine, no cracks. However there is a small leak between the cold start throttle blade shaft and the bushing. Now i have 4 TBs and another running rotary to use for reference and all of them leak too. Also, There is a slight leak into the manifold from the injector air bleeds that draw air from the same set of hoses as the AWS. I used a cap with a schrader valve where the AFM goes to pressure test the whole intake track and then sprayed everything with soapy water to find these.