20 replies to this topic

[ECOBRAP]

Hey all,

 

This whole year my 1.6 has been backfiring like I have never heard on any car. Whenever I let off the gas at high rpms it just pops away until I get back on the gas, see video below. I'm not too worried about it, I just have to go out a bit heavier because the car is burning a lot of fuel. I am also slightly concerned that this much combustion in the exhaust manifold might cause it to crack in the long run?

 

I have already checked for exhaust leaks. The car also idles very inconsistently at idle, rpms dropping to 300 and back up again to 1000, which makes me think the car is just running super fat. The AFM was adjusted a lot on the dyno at the beginning of the year, but I don't want to mess around with it because the air/fuel is perfect at full throttle.

 

It's fun entering every corner sounding like an assault rifle, but curious if anyone has suggestions.

 

Best,

Matt

 

 

[Bench Racer]

James, is it ok if I ask Matt a question about his issue? I may have had a similar experience with my 1.6. Does the idle rpm oscillate between 300 and 1,000 rpm all by it's self?

[ECOBRAP]

James, is it ok if I ask Matt a question about his issue? I may have had a similar experience with my 1.6. Does the idle rpm oscillate between 300 and 1,000 rpm all by it's self?

 

I don't see a comment by someone called James, deleted? But yes it tends to do it all on its own. Usually after sitting at a normal idle for a bit, it will then start to hunt and almost stall back and forth.

[Bench Racer]

Matt, my experience was similar. When let off throttle into turn, backfire widely. I did another lap and same backfire. In paddock, I slowly pressed the accelerator and as the rpm reached 2,200 rpm the rpm would drop to 1,700 rpm and oscillate back and forth. Checked as many things as I could think of back home in the garage. Vacuum leaks, electrical checks of parts and whatever. Then I found with the engine cold, the oscillation would not occur. After the valve cover/radiator was hand warm, the oscillation would begin. I knew during cold engine some sensor knowledge didn't feed to the ECU during this cold open loop time period. Check FSM, I no longer remember the open loop sensors that don't feed the ECU. Anyway I had all ready did the electrical check of the throttle position sensor per the FSM and it Ohms tested ok being shimmed per the FSM. The throttle position sensor is one of the sensors that doesn't feed the ECU during open loop. I disconnected the throttle position sensor and the oscillation would not occur when valve cover and radiator were hand warm. Bought a new throttle body sensor, issue solved. As I was typing I remembered typing this info in the past.

 

Hope the info helps.

[ECOBRAP]

Matt, my experience was similar. When let off throttle into turn, backfire widely. I did another lap and same backfire. In paddock, I slowly pressed the accelerator and as the rpm reached 2,200 rpm the rpm would drop to 1,700 rpm and oscillate back and forth. Checked as many things as I could think of back home in the garage. Vacuum leaks, electrical checks of parts and whatever. Then I found with the engine cold, the oscillation would not occur. After the valve cover/radiator was hand warm, the oscillation would begin. I knew during cold engine some sensor knowledge didn't feed to the ECU during this cold open loop time period. Check FSM, I no longer remember the open loop sensors that don't feed the ECU. Anyway I had all ready did the electrical check of the throttle position sensor per the FSM and it Ohms tested ok being shimmed per the FSM. The throttle position sensor is one of the sensors that doesn't feed the ECU during open loop. I disconnected the throttle position sensor and the oscillation would not occur when valve cover and radiator were hand warm. Bought a new throttle body sensor, issue solved. As I was typing I remembered typing this info in the past.

 

Hope the info helps.

 

When I get the car back I will see if it only does it when warm. If so I'll check all open loop sensors, thanks!

[Jamz14]

James, is it ok if I ask Matt a question about his issue? I may have had a similar experience with my 1.6. Does the idle rpm oscillate between 300 and 1,000 rpm all by it's self?

I was wondering if you were just obtuse or a-hole. Please proceed with Matt. You are incredibly talented at steering people in the right dire tion in answering their questions.

JAMES

[Tom Hampton]

I was wondering if you were just obtuse or a-hole.
JAMES

That's a false dichotomy.  It is quite possible (maybe even probable) to be obtuse AND an a-hole. 

[Jamz14]

That's a false dichotomy.  It is quite possible (maybe even probable) to be obtuse AND an a-hole.

True. I know I certainly can be both at times. But I am hoping the more charming parts of my personality outweigh the obtuse ahole parts most of the time.

[Tom Hampton]

True. I know I certainly can be both at times. But I am hoping the more charming parts of my personality outweigh the obtuse ahole parts most of the time.

I wouldn't bet on that on myself or anyone else on this forum. 

 

:-)

[Jim Venable]

Topgear Matt,

 

Did you resolve the issue and if so what was it?

[ECOBRAP]

The car is currently being painted so I don't have it. But I will post on here when I start messing around with it. My guess is still AFM but we will see.

[SaulSpeedwell]

"Tuned" AFM?

 

Some tuned AFM are just horrible - coughing, backfiring, smoke-spewing, etc.  But there also a lot of 1.6s that are much MORE horrible in terms of maintenance and being in specification on the rest of the idle system.  There are about 20 things that can go wrong that will make a tuned AFM cause idle cycling, high idles, etc.

 

A properly tuned AFM installed on a healthy 1.6 will likely result in a high, but steady, idle. 
Step 1:  Fully turn down the idle bypass screw at the throttle plate.

Step 2:  Adjust the throttle plate stop to the bare minimum.  This is a 3-handed job that must be done with the throttle at WOT. 

Step 3:  The pedal stop on the firewall must be adjusted to avoid "stretching" the throttle cable and twisting the throttle shaft.  I'll estimate that 80% of SMs that have been through an engine swap have this failure mode. 

 

*Tuned AFM Technical Notes*

Fast idle or idle cylcing is "normal behavior" on cars with other items out of spec or damaged  - but if the throttle position sensor, throttle plate position (idle stop), throttle plate air bypass, throttle dashpot, throttle pedal stop (on floorboard), and ignition timing -  are all correctly adjusted, the car should idle perfectly with a properly tuned AFM.  This cycling high idle is the Idle Air Control valve trying to compensate for a fast idle and since it is still too fast, the ECU cuts off the engine to avoid a runaway idle.  The cycling behavior will NOT affect torque, horsepower, or air-fuel ratio under race or dyno conditions.  But, the fast idle will potentially be "fighting" you under braking.    The shortest fix is to crank down the air bypass at the throttle plate, and adjust the throttle plate idle screw so that the throttle plate closes as far as it can without "sticking".  If this doesn't succeed at lowering the idle to 800-900, look for a twisted throttle shaft, leaking throttle shaft, vacuum leaks, or the throttle-anti-slam-closed dashpot holding the throttle open at idle.  (Note:  The dashpot gets "flexed" down in major crashes.  Look for witness marks on the underside of the hood to find out if your car has been in a severe crash).  The throttle pedal MUST bottom on the floorboard stop without stretching the throttle cable and twisting the throttle shaft.  Many cars have this screwed up from engine changes - because the throttle cable distance is affected by where the throttle cable is mounted to the intake manifold.

 

Fuel pressure and Adjustable Fuel Pressure Regulators:  Tuned AFMs that are not tuned on the dyno on your car assume "stock" fuel pressure of 37-39 psi, with a small margin to assure the motor doesn't run too lean.  Verify your fuel pressure and correctness of AFPR installation by monitoring fuel pressure on the dyno or track.
 
The other clue is to check the idle speed when fully warmed up, and then check it again with the TEN and GND pins shorted.  When everything is in spec, the idle will be the same.  If there is a difference, it means the IAC valve is having to work to keep teh idle down - and once the tuned AFM is introduced into this situation, it can produce fast idle and cycling idle.

[ECOBRAP]

"Tuned" AFM?

 

Some tuned AFM are just horrible - coughing, backfiring, smoke-spewing, etc.  But there also a lot of 1.6s that are much MORE horrible in terms of maintenance and being in specification on the rest of the idle system.  There are about 20 things that can go wrong that will make a tuned AFM cause idle cycling, high idles, etc.

 

A properly tuned AFM installed on a healthy 1.6 will likely result in a high, but steady, idle. 
Step 1:  Fully turn down the idle bypass screw at the throttle plate.

Step 2:  Adjust the throttle plate stop to the bare minimum.  This is a 3-handed job that must be done with the throttle at WOT. 

Step 3:  The pedal stop on the firewall must be adjusted to avoid "stretching" the throttle cable and twisting the throttle shaft.  I'll estimate that 80% of SMs that have been through an engine swap have this failure mode. 

 

*Tuned AFM Technical Notes*

Fast idle or idle cylcing is "normal behavior" on cars with other items out of spec or damaged  - but if the throttle position sensor, throttle plate position (idle stop), throttle plate air bypass, throttle dashpot, throttle pedal stop (on floorboard), and ignition timing -  are all correctly adjusted, the car should idle perfectly with a properly tuned AFM.  This cycling high idle is the Idle Air Control valve trying to compensate for a fast idle and since it is still too fast, the ECU cuts off the engine to avoid a runaway idle.  The cycling behavior will NOT affect torque, horsepower, or air-fuel ratio under race or dyno conditions.  But, the fast idle will potentially be "fighting" you under braking.    The shortest fix is to crank down the air bypass at the throttle plate, and adjust the throttle plate idle screw so that the throttle plate closes as far as it can without "sticking".  If this doesn't succeed at lowering the idle to 800-900, look for a twisted throttle shaft, leaking throttle shaft, vacuum leaks, or the throttle-anti-slam-closed dashpot holding the throttle open at idle.  (Note:  The dashpot gets "flexed" down in major crashes.  Look for witness marks on the underside of the hood to find out if your car has been in a severe crash).  The throttle pedal MUST bottom on the floorboard stop without stretching the throttle cable and twisting the throttle shaft.  Many cars have this screwed up from engine changes - because the throttle cable distance is affected by where the throttle cable is mounted to the intake manifold.

 

Fuel pressure and Adjustable Fuel Pressure Regulators:  Tuned AFMs that are not tuned on the dyno on your car assume "stock" fuel pressure of 37-39 psi, with a small margin to assure the motor doesn't run too lean.  Verify your fuel pressure and correctness of AFPR installation by monitoring fuel pressure on the dyno or track.
 
The other clue is to check the idle speed when fully warmed up, and then check it again with the TEN and GND pins shorted.  When everything is in spec, the idle will be the same.  If there is a difference, it means the IAC valve is having to work to keep teh idle down - and once the tuned AFM is introduced into this situation, it can produce fast idle and cycling idle.

 

Thanks for the info Saul. The car idles at 850, but occasionally will hunt between 200rpm and 850rpm almost stalling. If I don't touch anything, it will sometimes normalize and sometimes not. The car didn't do it last year, new engine put in along with different AFM that was tuned on the dyno. The car was running abnormally lean at low rpm so the dyno tuner fattened up the AFM a lot.

 

I think I need an Air/Fuel Gauge and Fuel Pressure Gauge to see what the car is doing when I am off the throttle. Checking open loop stuff today.

[ECOBRAP]

My fuel pump also seems to be making some noises I haven't heard before when the ignition is on but car is off, sounds like electrical stuttering like a computer CPU hard at work. Could this be related? Inconsistent fuel delivery when there isn't much pressure in the system?

[ECOBRAP]

The idle behavior on my car is similar to what's seen 53 seconds into this video.

 

Need to consider if backfire and idle issues are related or separate.

 

[Bad Rusty]

Young Mr. Cresci,

 

I have an idea.  You have a crazy fast 1.6 Miata and a super strong Rebello Motor.  Solid car combined with a ton of driving talent.  Quit worrying about how it sounds off the gas:)  Simple Cure, DONT LIFT!

 

Good luck in Fontana if you go, keep your headlights down...

 

And join us at Laguna May 23/24, I'll have the rust off (insert 1:45.9 here

[ECOBRAP]

Young Mr. Cresci,

 

I have an idea.  You have a crazy fast 1.6 Miata and a super strong Rebello Motor.  Solid car combined with a ton of driving talent.  Quit worrying about how it sounds off the gas:)  Simple Cure, DONT LIFT!

 

Good luck in Fontana if you go, keep your headlights down...

 

And join us at Laguna May 23/24, I'll have the rust off (insert 1:45.9 here

 

Not worrying, just curious why my car sounds like it has turbo anti-lag  . Yes I am headed to Fontana, been training on the simulator so I should get up to speed by the first race. Can't make NCRC, but I'll let you test the "don't lift" strategy through Turn 6 first! lol 

[SaulSpeedwell]

Thanks for the info Saul. The car idles at 850, but occasionally will hunt between 200rpm and 850rpm almost stalling. If I don't touch anything, it will sometimes normalize and sometimes not. The car didn't do it last year, new engine put in along with different AFM that was tuned on the dyno. The car was running abnormally lean at low rpm so the dyno tuner fattened up the AFM a lot.

 

I think I need an Air/Fuel Gauge and Fuel Pressure Gauge to see what the car is doing when I am off the throttle. Checking open loop stuff today.

 

200-800 rpm is a different story than the usual one we see with tuned AFMs.  What spark plug heat range? 

 

Is your dyno tuner an SM guy, or just some guy?  Low RPM is meaningless if you are talking about RPM lower than what you ever run on track at WOT. At part throttle the closed loop will keep you safe.  

[SaulSpeedwell]

The idle behavior on my car is similar to what's seen 53 seconds into this video.

 

Need to consider if backfire and idle issues are related or separate.

 

 

CA emissions car?  A/C car?  What is your coolant temp when the behavior starts?  That video (which is apparently a supercharged car) is looking at problems once the fan comes on.

 

Repeat:  "The other clue is to check the idle speed when fully warmed up, and then check it again with the TEN and GND pins shorted.  When everything is in spec, the idle will be the same.  If there is a difference, it means the IAC valve is having to work to keep the idle down - and once the tuned AFM is introduced into this situation, it can produce fast idle and cycling idle."

 

But check the timing and coolant temp when the behavior starts.

[RazerX]

Thanks for the info Saul. The car idles at 850, but occasionally will hunt between 200rpm and 850rpm almost stalling. If I don't touch anything, it will sometimes normalize and sometimes not. The car didn't do it last year, new engine put in along with different AFM that was tuned on the dyno. The car was running abnormally lean at low rpm so the dyno tuner fattened up the AFM a lot.

 

I think I need an Air/Fuel Gauge and Fuel Pressure Gauge to see what the car is doing when I am off the throttle. Checking open loop stuff today.

Matt, did you swap the AFM on the car?   Is so the one on when you purchased it was adjusted by Mr SpeedWell.   Are you still running the 5X racing AFPR?  Perhaps something to consider when you get the gauges hooked up.  

 

You can also look at the butterfly value with it closed and test WOT position to ensure as suggested it hasn't been damaged by pressing down to hard when you need extra HP...