“Baby steps.”  That’s what my mate has kept saying as I’ve been troubleshooting my new ride, a 1991 Mazda FC3S RX-7.  I’ve been working my way through the factory service manual, testing everything and hoping it was way out of spec, which would indicate the problem lies in that area.  But it’s been a long slog, with most things I’ve checked coming right in spec, or just out but not enough that it would cause an issue like an engine not to start at all.

But in the past week I’ve had two pretty significant successes.  I managed to borrow a fuel pressure gauge and found out that the car just wasn’t making any fuel pressure at all.  Alarms bells sounded straight away and I would have put my money on a faulty fuel pump.  So I borrowed a known working fuel pump and fuel pump assembly and put it in.  I also did a rewire modification for the fuel pump to make sure that a full 12V is getting to the pump and not less through almost 20-year old wiring.  This is a common mod for DSMs and early Evos, but I didn’t know until recently that FC owners do the same.  Got the fuel pump pumping and pressure was built up almost instantly.  So I figured it must be the pump that was bad.  Just to make sure I fitted my old fuel pump up to the new assembly, tried it and the darnedest thing happened.  Pressure built up again as it had been doing before.  So then I switched it and used the new pump with the old assembly and no pressure was building up.  Fuel pump assemblies going bad are almost unheard of so I was pretty surprised that it seems to be the case here, although my shock at this car seems to be diminishing with each thing I find.

Right, so I’m getting fuel pressure and the pressure is right where it should be according to the factory manual.  I put the intake manifold back on, turn the key and… it fires up!  And then promptly dies.  Tried again and the exact same thing happened.  It would start, go up to about 1,500rpm and then promptly die with a fair bit of misfiring.  If I put my foot down I could hold it at 3,000rpm and it would hold steady there in neutral with a few misfires but nothing too severe, but then when I took my foot off the accelerator the revs would come down and the engine would die again.  But I was a lot further than before and it was the first time the car had actually started without the aid of starter spray in around 6 months.

But what was making it die?  I checked everything was hooked up and connected, which it seemed to be.  The throttle position sensor (TPS) is infamous for going bad in these things, but me and a friend messed around with that for a while but came to nothing.  The only problem is that the sensor usually has to be adjusted when the car is fully warm, and I can’t really sit in the car and keep it at 3,000rpm while it warms up; I have to have a little consideration at least for my neighbours and the exhaust is backing right into a wall so it’s echoing beautifully!

So I went back to basics and thought about it again.  Revving the car higher got some stability, but it just didn’t want to run low down.  The 2 conclusions I came to were either the fuel injectors not firing enough for some reason, or a vacuum leak somewhere in the system.  I was lucky enough that a friend had a set of FC injectors spare and was willing to let me borrow them to put into the car and see if they would work.  I also took the chance to cut down on a little of the rat’s nest of hard and soft hoses in the engine bay too.  I plugged some nipples and rerouted a few lines (more of which I’ll do later down the line).

Turned the key again and… quelle surprise, it started and died.  Sounded a little different though so I fired it up again but it did the same.  I’m a strong believer in superstition so I gave it one more turn of the key and it started, and stayed started!  The idle was bouncing around between about 750 and 1,500rpm and not stabilizing but it was idling on its own without any pressure being applied to the accelerator.  Not sure whether it was the injectors or the vacuum lines I’d changed on the car, but it seems to be teetering on the brink of running now.  I’m pretty sure the TPS is out of sync which is causing the strange idle, but once that’s sorted out I should be good to go.  I’ll also clean up the engine bay a little more and simplify things as much as I can without losing any drivability or performance.  My own injectors are going to get sent off to Witch Hunter in the States to get cleaned and flow tested too, to see if they were at fault.

So while baby steps take their time, they seem to be paying off in the long run.

Tags: car problems, Cars, fc3s, featured, rx-7. rotary, wankel

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