That is true, but when you do a rebuild (like what they stated was just done) the compression can be low for the first bit while the engine is breaking in. As a minimum, 500 miles should be allowed for break in keeping the engine rev's low, <5000. During this time the compression in the engine will improve.

Wow... umm... in english please?

I think you are assuming that there is something wrong with his water temp sensor, which it could be but that's a big assumption to make based on the information at hand. I'd say since the check engine light is coming on he should to check to see which code the ECU is throwing before he replaces anything.

Finding out what the ECU says is wrong will get you started in the right direction. I think FC3Spro.com has instructions on how to check the codes, or it can most likely be found in the FAQ section of the forum.

The other option that might work, and i'm sure people on here will jump all over me for this, is you can adjust your throttle stop in so that the car idles at about 1500rpm. Doing this should allow it to idle while the engine is breaking in and then once broken in you can play with it and adjust it to where you want it. I would still check the code first though.

 

We checked all the codes last week. We had about 8 or so show up. Half of them were due to the emmisions removal. We went over everything and even replaced a few things. It seems to be a little better but it still is having hot start issues, and wants to stall at anything past 1/4 throttle.

 

How many miles are on the engine since the rebuild?

At about 1/4 throttle is about where the short range TPS stops and there is only the long range on the S5 motor. I would expect the ECU to throw a code for a bad or mis-adjusted TPS if that is the case but based on what you are putting here i'd say that could be a place to start.

I'd also check for cracked vaccume hoses and vaccume leaks. They can cause all kinds of funny things to happen.

 

100 miles on rebuild. We will swap out the TPS this weekend. No vaccume leaks at all.

 

Thanks guys for this help!

 

Turns out the OMP was faulty. Still won't start when hot though.

 

ooo thats why it didnt go past 1/4 throttle it was in limp mode...i actually forgot about that....when you rebuilt your engine did you get your injectors cleaned and matched?

 

No that is one thing I would like to do alomg with a new set of O rings for the injectors. Although it seems to be fine with plenty of throttle response when it runs good.

 

Horray for codes!!! lol. I forgot about that possibility too.

Get the OMP fixed and get about 500-1000 miles on the engine. I wouldn't be surprised if your hot start issue all but goes away.

 

Just check the voltages at the ecu to see if they're in range. I had the same issue with my rebuild, turned out that even though the water temperature sensor (itself) was in range, I was getting 5v at the ecu. I checked the wiring for continuity, and found no breaks. Weird.... so after much fighting with it, I found that the wires were pulled out of the back of the connector for the temperature sensor just enough to where they weren't making contact with the sensor. The failsafe mode was 70deg F, so when it was hot it was DUMPING fuel. Would not start at all until the car cooled completely down.

Low compression is not always the culprit of a flooding engine.

 

don't mean to steal but my car has similar symptons and its for sure not my injectors. i'm running on carbs and its a fresh rebuild like 800miles?!?

 

What made u check the omp?

 

It has happened before. The S5 ECU is tricky It won't run right if the omp is unplugged or if there is something wrong with the mop.

 

Carb's are a whole different ball game from EFI when it comes to symptoms and diagnostics like this. I think with a Carb it will be a fuel volume issue, so pump, filter, regulator, and the carb internals could be the cause of the restriction, and there could be many other things causeing your problems too. You should probably talk to the 12a guys in the first gen forum.

 

Fail safe mode for the failure of a water thermo sensor is approx 180*F.

 

So if someone could break it down for a new rotary guy like myself....why does a new rebuild lack compression to start reliably???? Seems that a rebuild would improve compression and reliability???

 

Old parts have a certain amount of wear and tear on them. Mazda has specifications which account for this little bit of wear when deciding whether a part is re-useable or not. A used part that is within spec is not perfect, it's just close enough that it considered to be ok to use.

When a part starts to wear, it never wears perfectly evenly. Quality new parts are fabricated to within very tight spec. Usually any imperfection is so small that it is not noticable when you look at the part. However, when trying to compress ari/fuel the mix of wear on the used part and the imperfection on the new part may not make very good compression. This is because both parts will have slightly high and low parts when they press together to make a seal. Natually, the high parts will wear quickest and as they do more of the seal will actually touch what it is sealing to, which will equal better compression.

Exactly how long it takes a rebuild to fully build compression (the apex seals to seat) depends on the condition of the used housings and the kind of apex seal being used. Some will have good compression almost right away and some will take quite a while before they have good compression.

As for the hot start issue, I really don't know an explination as to why a rotary with low compression won't start while hot, but they do. I'm not sure if there is an exact number where someone can say for sure there will be a hot start issue or not, but I know when I got my car it had 90psi front and 60psi rear (thanks to 2 chipped apex seals and 2 cracked corner seals) and it would flood EVERYTIME I tried to start it with the engine hot. I don't have compressions number post rebuild, but I never had a hot start issue after the rebuild so I assume the compression is good.

90psi is a general number too. Exactly what condition an engine compressing to 90psi is in depends on the engine. A S5 N/A (9.7:1) that is compressing to 90psi is showing more wear than a S4 Turbo (8.4:1) (I think thoes compression ratios are right but i'm too lazy to look them up). Either way, compression will be lower than normally desireable. However some engines can last quite a while on 90psi.