Hi everyone,
I recently had my engine rebuilt, and I'm having two problems. One is that when I start the engine, the RPM needle is very unstable, it drops to 0 and then goes back up.
The other is that once the engine warms up, it idles very roughly. What could be causing this?

 

Be careful clicking on links from new posters.

 

?? I don't understand what you mean

 

@despistao your link looked a little odd so he was concerned about it. try posting a better link to youtube.

 

are you saying with the first that it regularly drops to zero while idling or just once during start up?

 

Scammers are continually joining with new names and trying to get members to click on links, that can turn out to be "phishing" links.

Some try to build up a posting history to try to look like bonafide members.

Posting up a vague request for help is rendolent of such attempts.

 

uploaded the link to YouTube, from the mobile it wouldn't let me

 

That's it, as soon as you start cold, the needle goes up and down suddenly (the engine does not change its rpm) only the tachometer. Maybe some sensor?

Also, when rebuilding the engine, a wiring harness from Wiringspecialites was installed.

 

I understand your distrust, but this is not the situation, I`m not a scammer

 

looks like a misfire... do normal troubleshooting and check the basics

 

The tach gauge itself may be malfunctioning. This is a common problem for FD. The tachs can be repaired.

The problem is sometimes exaggerated by cold, or freezing, outside temperatures.

 

Is your car running a stock ecu or an aftermarket one?

If you have a Power FC, for instance, you can check the revs on the controller.

 

eco stock, The whole car is OEM, except I've disabled the EGR valve.
I could maybe look into a PFC, but I don't know who could tune it.

 

I think the basics are checked.
New spark plugs,
coils OK,
coils wiring OK.

 

now I'm uploading a video

 

I wouldn't add anything until you determine the problem. It was asked because that adds a whole other potential problem if it's modified.

 

ok, My intention continues to be to keep the car all the stock possible

 

Nasty misfire.

Newly rebuilt motor (like brand-new? -- like just put it in?)... bad misfire... poor idle... bad tach...

Any number of things could cause them independently, but altogether, happening all at once... I'm leaning towards a bad ground maybe?

 

Complete rebuild: housing, apex, seals, etc.

The tachometer only does it when cold.

Hmm, bad ground or bad connection? Now who knows which!

 

Sorry, by 'new' I was poorly referring to how long ago it was put in. Ground issues certainly can, but don't often pop up out of the blue. Not in a short time anyways. They usually pop up right away after a big job or after many, many years of aging. So if it was running fine for weeks or months, not quite so likely. If it was running like this right after you put the new engine in, then it's more likely.

The FD has notoriously shitty OEM sensors -- exacerbated after decades in overly-hot engine bays. Throw in 30year-old capacitors and all bets are off on our electrical systems. So the tach thing could be completely unrelated and just it's deciding to go on the fritz. But happening at the same time as misfires makes me think grounding issues.

 

I haven't double-checked this myself, but the factory wiring diagram says that the tachometer ground comes from 'ground point 3' on the engine harness, which is a ring terminal that bolts to the upper intake manifold near the firewall. It's a spot that would be easy to miss if you're not careful. That part of the engine harness is somewhat near the downpipe so it might have gotten brittle with time and heat. The ground signal wire for the tacho also goes through connector X-14 which is one of the rectangular connectors inside the cabin behind the dash near the ECU. There are at least two of those connectors in that area that usually get disconnected when removing an engine, so it would be a good idea to look up in that firewall/dash/ECU area to make sure everything is connected.

If you're confident the tacho ground is good, I would take a close look at the CAS (crank angle and cam angle) connectors and sensors, and their brackets, and the trigger wheel on the crankshaft pulley. The ECU sends the engine speed signal to the tachometer, and it measures engine speed using the two CAS sensors near the crankshaft pulley. If those wires have problems, or the sensors are not mounted securely, or the crankshaft pulley is not installed correctly that could cause problems.

Other problems I've heard that cause rough idle could be a MAP sensor vacuum line unplugged, or the MAP sensor itself not plugged in to the harness. The coolant sensor is the same plug as the fuel temperature sensor, and the harness is just long enough that the plugs could be connected to the wrong sensors. That would give the engine the wrong fueling which could make it run rough.

I'm surprised Redbul hasn't mentioned spark plug wires, if those are not installed properly the engine might run rough. It's hard to see that area without removing the intake pipe near the throttle, but the wires from the two-post leading coil need to connect to the lower (leading) spark plugs. The leading wires can go to either front or rear rotor, with the stock two-post leading coil both the front and rear leading plugs fire at the same time. The single-post trailing coils go to the top (trailing) spark plugs. As a test, you could disconnect the trailing spark plug wires and see if the engine runs any better or worse. They shouldn't make a huge difference, and having the trailings connected should not make the engine run rough.

Edit, here's a video showing that ground location on my car:

 

The car is LHD, so the USDM FSM can be used as a guide for the hooking up of spark plug leads and the coil harness.

I usually chime in when the car is JDM Version 4 or newer, because Mazda changed the mounting order of the coils, and if an owner is unaware of that, they could hook things up the old way, and cause big problems.

(The coil harness for the LHD cars is a bit complex, with its own wired in grounding lines - probably best to replace. Note that the newer JDM coil harness is more simple, but will not work well on the USDM set-up. Make sure you have the correct coil harness.)

Meanwhile, it is very brave to try to retain the original set-up with all sequential twin turbos and all the emission stuff.

A very large number of owners will have given up that effort long ago.

There are just too many gremlins. Also consider that your stock ecu may be over 30 years old.

One of my first questions, these days, is, "how old is your gas?"

Ethanol in the gas these days means it attracts water if sitting too long.

Next, is the check engine light on?

If yes, don't ignore it, you need to check codes.

There may be up to 50 different malfunctions the codes can make you aware of, and save you going through the engine bay checking all the solenoids, sensors and other components one by one.

Even though it is a new motor, run a compression test.

The dash wiring has two grounding points, those could affect the gauges.

There is new diagnostic software for PFC, that may be able to analysis and essentially self tune the PFC to your set up.

 

In certain cricumstances the ECU will put the car into "Limp Mode". The ECU might not like that you removed the EGR valve.

 

Yes, this happened after removing the engine and rebuilding it.
The engine harness is new from WiringSpecislites.

Also, the coil harness is the latest model, which doesn't have a ground connection.
Perhaps I should look for a coil harness with a ground connection?

 

thanks!

I'll check all the ground connections you mentioned, in case any are disconnected or causing a bad ground.

I'll also check the coolant and fuel line connectors, and the MAP sensor.

I'll also carefully review the video you attached.