Greetings. My car jerks/bucks a lot and backfires when I try to boost it. Also when I try to press the pedal more than 10-15%.

The car had some boost issues for many years. The boost pressure was low but it used to run fine with a little bit lack of power. I have removed the air pump and the BAC valve and sealed off the holes.
Recently I changed the following parts:

New injectors
New radiator
New oil radiator
New spark plugs
New TPS
New oxygen sensor
Rewired the air intake sensor because the pins broke when I removed the intake.

I have ckecked for vacuum leaks and It seems fine.

The car starts fine even when hot, holds a perfect idle at 900 rpm but as I said it bucks, jerks and backfires when I try to boost it or when I press the pedal more than 10-15%.
The funny thing is that the car boost fine only for once when I start it cold but when I try to boost it again it starts to jerk.

I have noticed that the Air/fuel ratio gauge that I have installed shows lean when it jerks and backfires.

I have thought to ckeck the fuel pump but I dont know how. Also I want to check the AFM but I dont know how to check it either.

Any ideas or suggestions?

Thanks in advance.

 

I'd double check for vacuum leaks by spraying carb cleaner in different spots around the engine while its idling. If the idle changes you have a vacuum leak.

Are you on the stock ECU? Does the condition happen after a certain RPM? Checked fuel pressure? Changed the fuel filter? Original fuel pump?

You can rent a fuel pressure gauge from autozone or oreillys.

 

Stock ECU. It happens at any RPM, when I press the gas pedal more the 10-15%. I changed the fuel pump many years ago but have not checked it since then.

Thanks for your reply

 

Check both your TPS.

 

If you have access to fuel pressure tester you could install one inline to the fuel feed hose on the left rear of engine. This way you can see what fuel pressure is during bucking stalling condition. Would help give direction in you diagnosis. I know that the plastic fuel sock/cage on S5 fuel pumps becomes restricted with debris and can cause a lean, bucking, stalling loss of power symptom just like yours. Wouldn't take much to pull the pump and inspect fuel cage for debris

 

As I said the TPS is brand new. I just installed it today. Does it need to be checked if it is brand new?

I will check the in tank fuel filter and try to find a fuel pressure tester.

Thank again for your replies. Any other suggestions or ideas are welcome.

 

You replaced both TPS? Did you check the resistance? its dependent on the throttle position. When was the last time you changed the fuel filter?

 

The fuel filter is really old...more than 10 years ago...
But the TPS is brand new an OEM replacement, I dont think that it needs to be tuned, right?

 

The TPS certainly needs to be adjusted when installed, the image posted describes how.

 

Really??? Then this must be the answer!!!! I thought that you just install it. Thanks so much. I know how to adjust it. I tried to adjust the old one and it read 6.3 with WOT.
I will try to adjust it tomorrow.

 

Be sure to follow up.

 

Dont assume this is the answer. I am suspicious of a few things. And just throw a new fuel filter on it. It’s like $15. Boost pressure sensor working correctly? Coolant temperature sensor working perfectly?

and I’m assuming the check engine light isn’t on/flashing since you didn’t mention it.

 

hi all, today I spent the whole morning trying to find out what the heck is going on...

I started by checking the TPS. I did NOT use the method with the two multi-meter probes that are inserted in the back of the plug like in this photo, because I was afraid that I will damage the plug. The probes are too big to be inserted there. Where do I have to put them? Between the rubber and the cable or between the rubber and the plastic of the plug?



So I just used the multi-meter to check the resistance on A-B (narrow range) and E-D (full range) like the FSM page posted by FührerTüner.
The car was warmed up properly. Both measurements are within the proper range. So I guess that my brand new TPS is not the problem. Am I right?
Next I noticed that the oxygen sensor cable that goes to the plug had been disconnected somehow and I connect it again.

Then I did a test ride and although that the car boosted perfectly for 5-6 times and pulled really nice drifting the rear, then all of a sudden started to buck/jerk heavily again when I press the pedal for more than 10-15%.
I stopped, turned off the engine, started the engine again and tried another test run. Exactly the same result. The car boosts fine for 3-4 times and then starts to buck, jerk and backfire.

So then I went to the mazda dealer and used the self diagnosis checker. Now the only code that I get is code 07 which is the boost sensor code right?
I ask that because in this page says (zenki 86-88 FC) that It is the boost sensor :FC3S Pro v2.0:&nbsp How-To - Zenki FC3S Error Codes
But in this page (kouki 89-91) says that the Pressure Sensor (intake manifold pressure) is code 13: 89-91 CPU Trouble Codes
And in the 89-91 turbo FSM Fuel and emmisions (page F2-20) the 07 code does not exist!!! : http://www.rotaryheads.com/PDF/2nd_g...tems_turbo.pdf
So does anyone know what is the 07 code?

Any ideas on what can cause the boost problem after 3-4 perfect acceleration runs? Does the car go in safe mode somehow?
Can it be an fuel cut problem? I have a full turbo back motoria exhaust with 80mm donwpipe and a HKS mushroom like intake that goes straight to the turbo.

Any help will be highly appreciated guys. Like the titles says...I am really desperate!!!

I

 

Also I would like to mention that I have installed an HKS FCON parallel ECU but I do not use it all. The dial is turned to 0 so according to the manual it does not interfere with the OEM ECU. The HKS FCON is installed for more than 10 years but never used it as I said.

 

You should change your fuel filter. Its probably not the problem, but its a possibility considering its probably never been changed. You should also pull your fuel pump and check the sock. Then you should check fuel pressure.

 

You mean the big fuel filter in the engine bay? Is there a way to check it out? I think its really difficult to find one where I live. Do you know what part number does it have?

 

Any auto parts store should be able to look up a new fuel filter.

If not, i would order one.

If your fuel system is that old and hasn't been serviced....I would highly recommend starting there, as that's a recipe for disaster for any car.

Pull your fuel pump and inspect the tank and filter sock too.

 

Ok I will. But the car was fine before all the new installations that I describe above. Also the strange thing that it boosts fine for 3-4 times and then starts bucking. That's why I dont think that the boost problem is related to the fuel system.

 

On the injectors, are they aftermarket or?

Might check your spark plug leads to make sure they're hooked up properly.

 

They are aftermarket bought from ebay. Like these ones but 550cc, not 650cc
https://www.ebay.com/itm/4-x-650cc-D...gAAOSw3tpalILG

These kind of injectors are similar and do not require a certain order to be installed, right?

I think I will check the spark plugs again when I will change the oil filters.

Does anyone know what sensor can cause fuel problems except TPS and intake temp sensor?

 

I wouldnt trust those chinese offbrand injectors. Yiu should have just got the old ones cleaned. With that being said you should check the impedance and make sure theyre compatible with your harness, whether or not your running resistor packs.

Edit: i just saw you have a 91 so impedance shouldnt be and issue, but id try swapping the old ones back in. You cant trust those injectors.

Another thing is those injectors are rated at 550cc @ 3 bar which is 45psi. IIRC FC fuel pressure isnt that high.

 

Unfortunately the old injectors are worn off beyond repairing and cleaning....
Where did you saw that they are rated @3 bar? I had installed 7-8 years ago an aftermarket fuel pump which had like 280lt/hour capacity I think it was a denso or other popular brand. Do you believe that it not enough for these injectors?

Also if the problem was caused by the injectors wouldn't that affect the engine all the time? Why does the car boost fine for 4-5 times and then starts to buck and jerk?

 

Wait a minute. You said everything ran fine until some recent changes including new fuel injectors. You said the car ran fine before. So how are the old injectors bad then?

The ebay link you provided, in the item description its states @ 3 bar.

Who knows what could be happening with those injectors. Maybe after a couple pulls theyre heating up your injector drivers in you ecu. You shouldnt trust some chinese crap when dealing with critical components such as fuel injectors, especially with these delicate engines.

 

The guy who sells them, has sold many kits and has no complains in his feedback page. The old injectors were at really bad condition. They were the stock injectors and the car has run more than 160.000 km with them.
What do you mean when you say that the injectors heat up the drivers? I don't think that this is possible.

 

Lol ok man well you already know everything so good luck!