13b doesn't like to start when hot

mbuskuhl

mbuskuhl

New member
Looking for some advice.... The car does not like to start when hot (over 200 degrees). Normal? What's the problem? Do I need an elec fan?

The car has a Ron Davis radiator (distilled water and water wetter), ducted creatively, splitter, dual pulleys, 160 degree thermostat. It doesn't get over 210 when racing (and that's a 90 degree + day) no problems there. Normally it runs around 200. No problem on cool down lap either.

Temps will climb quick sitting on grid (I try and keep the motor off usually), sitting off course waiting for a tow or in the pits after a race (I pop the hood to stop that).

For example, I ended up in a gravel trap and needed a tow, I killed the power but temps still rose to 230 and then it wouldn't start after getting pulled out until it cooled back down. I have been on grid idling thinking we are about ready and then a delay happens so I cut the motor and temps sit high and then I can't restart.

The motor has less than 20 hours on it. How can I make sure it starts when hot?
 
Might be an issue with heat soak on your intake air temperature sensor(s). Hot air is not as dense, and needs less fuel to get the same air/fuel ratio. The ECU puts less fuel through the injectors when the air temps sensors indicate hot, leaning out the mixture. In the RX-7 (along with many other cars) with a hot engine this/these sensor(s) heat up when there isn't air flowing to keep them cool. It might be making the mixture too lean to start. I'm not sure how to legally fix it with a stock ECU.
 
Thanks for the responses so far. Yes the starter turns over. I figured it's some kind of heat soak but wasn't sure if all 2nd Gens experience this.

Do the rest of you have this problem? If so, do you fix it with an elec fan or ? Is this more of a me rather than 2nd Gen issue?

Since you mention fuel, I've got an FPR set at 30 PSI and it is an S4 motor. I don't know if that has much impact. When the car cools under 200 it will start again, I have got it to start at 210 but usually it just cranks and cranks when over 200.
 
You probably know this but the 13b has a fuel-cut built into it. This is to keep it from flooding when it's hot. Basically if you floor it while engaging the starter (at least with the stock key and such) the fuel pump will cut.

I would bet that if you tried that it'll start right up.
 
First thing you need to do is an accurate compression test. Hot start issues are the biggest sign of low compression. That said I doubt that is the problem since it will start below 200.

Marty might be on to something. Fuel pressure spec is 34-39 (no vaccum applied to the regulator) so you are already lean if you are only running 30.

Get a fan on that thing. 230 is enough to begin cooking seals.
 
I did not know about the fuel-cut but I do not floor it when starting hot, in fact I don't touch the pedal as I don't want to flood it. I usually always start it without pushing the pedal. I can also always see my fuel pressure as the gauge is on the hood to make sure I've got pressure. I did this after a line got kinked before and since my FPR needed a gauge, why not the hood.

I'll try a compression test as this has not been done. It was also on the dyno at 167rwhp so I don't think there is an issue. Even at the dyno there was not enough fans on the front, it got hot (215-220) and we had to wait for it to cool before it would start again, therefore I don't think compression would be an issue with that HP but will double check.

Fuel pressure is at 30psi as this gave the best HP and a A/F of around 13.7 which I read to be typical.

It races within 190-210 but gets hot sitting and won't start. Am I the only one with this problem? For those that have fans, what do you recommend for a Ron Davis radiator? I might as well add that oil temps run right around water temps or just a little less, oil never gets hot, even sitting.
 
Are you sure it's the heat and not simply that the engine has been running for awhile? RX-7s are notorious for not starting easily when hot, but the problem may simply be that it is flooded. I.e. when you shut it off it did not burn the fuel already in the chamber and/or your injectors leak a bit. I always kill the engine by shutting off the fuel pump and allowing it to die; that way all the fuel is used up and you are not trying to start a hot engine that is overly rich. If you have your fuel pump switched, try that. Otherwise, try Scooter's remedy.
 
Are you sure it's the heat and not simply that the engine has been running for awhile? RX-7s are notorious for not starting easily when hot...
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I have been wanting to know if this is a common problem, looks that way, thanks for confirming. I'm going to look into doing a fan and wiring in a switch to cut the fuel pump.
 
Just noticed this in the S4 FSM this morning. Don't know whether the S5 is the same.

The S4 has a "Hot Start Assist" feature. You may know that the vacuum to the fuel pressure regulator is switched by a solenoid controlled by the ECU. The fuel pressure is reduced at idle by this switched vacuum. The idle mixture is tuned using this lower fuel pressure.

The Hot Start Assist feature is that if the ECU senses a hot intake manifold air sensor at startup, it will leave the fuel pressure vacuum turned off for 90 seconds, thus richening the mixture. I presume that after 90 seconds the air flow should have cooled the air temp sensor sufficiently to provide an accurate reading.

So if you don't have your fuel pressure regulator connected to the switched vacuum you might try that.
 
Does your motor have carbon apex seal?

Not sure if it's the same with the 13b motor but my 12a with carbon apex seals always started hard when hot even when it was new but after that motor gave up and I installed a bone stock motor the car now starts fine hot.

Nothing else was changed it has the same cooling system and carburetor.
 
Rotarys like to flood when they're hot because of low cranking compression. Compression is lower when warm than it is when they're cold. Hence the very common hot start issue. A hot battery with lower voltage, hot wiring, hot starter all lead to slower cranking speeds as well which drops cranking compression even more. It doesn't fire on the first few revs while fuel is continually pumped in. Very quickly the chamber walls are washed down and the problem snow balls.

If it's flooding it'll foul plugs and then it will absolutely refuse to start until the plugs are cleared. If it cranks and cranks and cranks and then slowly sputters to life it's more likely cranking compression related. This doesn't neccesarily mean a bad motor. The faster the starter spins the motor over the better the cranking compression. A weak starter and battery can contribute to hot start issues. Do a hot compression test. Measure voltage at the starter while cranking.

Bill's tip on shutting the car off by killing the fuel pump is a good one. But that really goes for any car IMO.
 
You can pull the "relay cube" just under the steering column to cut the fuel pump and crank the car while you are in it. It is hard to reach unless you have long arms. Its connector is attached under the column down near the dash. You can cut the wiring to the pump here to add the fuel cut switch.
 
Does your motor have carbon apex seal?

Not sure if it's the same with the 13b motor but my 12a with carbon apex seals always started hard when hot even when it was new but after that motor gave up and I installed a bone stock motor the car now starts fine hot.

Nothing else was changed it has the same cooling system and carburetor.
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We had that problem with the 12A when it was cold took forever to get started I was told that was because of the carbon apex seals...however our second gen fires right up hot or cold. I reccomend an electric fan I run it on the grid and during the pace lap then shut it off during the race. It certainly helps cool things down after the session also.

On another note we have an S4 and found that between 40 and 42 psi on the FPR works the best. And like Chris said I always kill the pump to shut the engine off to relieve the preasure.

Bill Weaver
ITS #63
 
Thanks for all the suggestions. Hopefully I got this worked out, I have installed an electric fan which does a great job. I idled the car at home until it got to 220 simulating an on grid situation, then I flipped on the fan and bam - the temps started dropping fast, all the way to 190 where they held, what a much better feeling now. I also wired the fuel pump direct (it's not OEM) so I can kill the car by cutting fuel, and added a new battery while I was at it. This should do it. Maybe I'll get a new starter or figure out how to test mine out, I'm not sure how old it is.
 
Those changes may help, especially the fuel pump. I run mine with a switch and burn off the fuel in the lines before shutting it down. I can't tell you much about the 13.7 ratio, I run my mixture from an EGT and adjust fuel pressure till my EGT looks good ( nothing over 1750). If the fan helps use it, I have a fan, but only use it during a black flag.
 
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