67 replies to this topic

[RazerX]

Matt, i have an extra IR gun i can bring down on Thursday

[ECOBRAP]

I actually have one, and never even thought to use it. Maybe I'm a pyromaniac and naturally ignored it so that I could spray something on the exhaust manifold lol. I will use the IR gun on Wednesday when I do my next series of checks.

 

I have thoroughly looked for vacuum leaks, including carb cleaner, rpm did not move an inch.

 

I have been thinking it over, and I did not touch ANY engine supplements when the motor was rebuilt, just took them off and put them on when I got it back. The ONLY thing that I changed was the idle control valve under the throttle body. That is now the number one suspect, and if that is the problem it will be frustrating considering it is new.

[B(Kuch)Kucera45]

Did you pull the plugs out to see what they look like?

Like I said before, I had a bad ( new ) fuel injector that was stuck open and smoothed out when you come off idle and it sounded just like your video. This is a five minute check, just pull the plugs and see what they look like. If it's white it's lean and injector blocked and if it's soaked the injector is stuck open or that cylinder it's firing.

[Bench Racer]

If when you jumper(open loop) /un-jumper (closed loop) and the rpm increases to 1,100 rpm and back to idle speed, and back and forth as you jumper/un-jumper it would seem to be a clue towards the issue item.

 

Drago, Wheeler, engine builders, a little help.

[ECOBRAP]

Did you pull the plugs out to see what they look like?

Like I said before, I had a bad ( new ) fuel injector that was stuck open and smoothed out when you come off idle and it sounded just like your video. This is a five minute check, just pull the plugs and see what they look like. If it's white it's lean and injector blocked and if it's soaked the injector is stuck open or that cylinder it's firing.

 

When I pulled the old plugs (they had been in the motor for the first few heat cycles), they were black. I will do another test with the new plugs and see what they look like after 10 minutes if idling, and what they look like after 10 minutes of driving.

 

 

If when you jumper(open loop) /un-jumper (closed loop) and the rpm increases to 1,100 rpm and back to idle speed, and back and forth as you jumper/un-jumper it would seem to be a clue towards the issue item.

 

Drago, Wheeler, engine builders, a little help.

 

I agree, this is an interesting post. I find it difficult to set my spark timing because whenever I do the jumper, the rpms rise to 1100 which advances timing anyways. If I use the idle adjust screw while the jump is in place, and lower the idle, it stumbles too much to set accurate timing.

[LarryKing]

On a 1.6 intake manifold there is a vacuum tube on the back, below where the OEM injector loom bracket and ground attach. The tube points toward the firewall and is hard to see if you are not specifically looking for it.

 

Make sure that tube has a rubber plug on it.

[ECOBRAP]

On a 1.6 intake manifold there is a vacuum tube on the back, below where the OEM injector loom bracket and ground attach. The tube points toward the firewall and is hard to see if you are not specifically looking for it.

 

Make sure that tube has a rubber plug on it.

 

There is a new plug on that tube, all plugs are new and uncracked

[Bench Racer]

Off the wall question, is the butterfly in the throttle body closed at idle speed? A tad of slack in the throttle cable when you push down on the cable. There is an adjustment screw/nut in the cast aluminum TB housing that positions the "U" shaped bracket that fits on the butterfly shaft which will allow the butterfly to close on the through bore. 

[Bench Racer]

When you cleaned the throttle body, if you used carb cleaner you could have eliminated the material applied to the throttle body bore for sealing (per say) when the butterfly is closed.

 

Is there air passing/leaking through the throttle body gasket.

 

Something about the rpm increase (jumper) and the idle adjustment screw adjustment doesn't set well with me.

 

Use Google as a search engine and search for "1990 Miata ECU closed loop high idle" by Juan 11/22/09 and read the third topic down. It should be from the old Spec Miata site. Some good people, with good thoughts, read the entire thread all while thinking about your issue.

[ECOBRAP]

When you cleaned the throttle body, if you used carb cleaner you could have eliminated the material applied to the throttle body bore for sealing (per say) when the butterfly is closed.

 

Is there air passing/leaking through the throttle body gasket.

 

Something about the rpm increase (jumper) and the idle adjustment screw adjustment doesn't set well with me.

 

Use Google as a search engine and search for "1990 Miata ECU closed loop high idle" by Juan 11/22/09 and read the third topic down. It should be from the old Spec Miata site. Some good people, with good thoughts, read the entire thread all while thinking about your issue.

 

Considering the ICV was the ONLY thing I changed when putting the motor back in, I am going to put the old one back in and see what happens. I replaced it because of a high idle, but when I took off the intake manifold from the old motor, there was no rubber plug on the back tube, so the high idle may have been caused by a vac leak and I may have replaced the ICV for no reason whatsoever... and the new one may be malfunctioning.

[ECOBRAP]

I am currently in stage 10. Worth a good laugh in the middle of the week, check it out.

 

http://www.carthrott...ult-repair-job/

[Alberto]

Could the Throttle Position Sensor be mis-adjusted?

[ECOBRAP]

Could the Throttle Position Sensor be mis-adjusted?

 

That is on the agenda of things to check!

[ECOBRAP]

Update:

 

Did a compression test, even across all four cylinders.

 

Checked exhaust manifold temps, even across all four.

 

Pulled spark plugs, number four spark plug is black and smells oily (suggestions?)

 

Confirmed all plugs are sparking.

 

Put a screwdriver onto each injector, held my ear up to it. I could hear the injectors firing on/off consistently.

 

 

I need to get this figured out this weekend, heeeelp lol.

[Bench Racer]

Oily # 4, this is a newly rebuilt engine correct.

 

In absolute dark in garage with door open when running rough look under hood for sparing at coil and plug wires.

[ECOBRAP]

Correct, new engine. Just checked in the dark, everything looked fine.

[B(Kuch)Kucera45]

Oily could be fuel.

Just because you can hear the injectors firing doesn't mean they are ok. That one could be stuck open allowing fuel to dump into the cylinder when it should be off. If you have any other injectors replace that cylinder that looks bad, I'm almost sure that is your problem.

If you don't have any spare injectors switch them from one cylinder to another and see if the color of the plugs change. This very easy and will only take you about 15min.

If you had oil in that cylinder it can be caused by a bad valve stem seal or bad rings or ring alignment but should show up with a compression test. So I think the injector is stuck open,switch with a new one or you can check it by switching it with another cylinder and then check the plugs again. If it changes cylinders it will show on the plugs. Like I said,I had one doing the same thing. You can hear it firing but you have no way of knowing if it shut off all the way.

If you keep this up I'm going to get the broken record award from Jim !

[David S.]

Have you checked cam timing?

[Steve Scheifler]

If as stated all the exaust manifold tubes are the same temp then the spark plug may be an oily red herring with respect to the main issue, but still needs to be resolved. Also, I don't recall it sounding like it was running on 3 cylinders which is quite distinctive and would be obvious when you drive it.

[ECOBRAP]

Oily could be fuel.

Just because you can hear the injectors firing doesn't mean they are ok. That one could be stuck open allowing fuel to dump into the cylinder when it should be off. If you have any other injectors replace that cylinder that looks bad, I'm almost sure that is your problem.

If you don't have any spare injectors switch them from one cylinder to another and see if the color of the plugs change. This very easy and will only take you about 15min.

If you had oil in that cylinder it can be caused by a bad valve stem seal or bad rings or ring alignment but should show up with a compression test. So I think the injector is stuck open,switch with a new one or you can check it by switching it with another cylinder and then check the plugs again. If it changes cylinders it will show on the plugs. Like I said,I had one doing the same thing. You can hear it firing but you have no way of knowing if it shut off all the way.

If you keep this up I'm going to get the broken record award from Jim !

 

Thanks Kuch. After today's experiments I am almost positive that must be the problem. I replaced the idle control valve with the old one, no difference.

 

I don't understand why an injector would have gone bad, considering they were all fine two months ago when I took the intake manifold off the old motor, but I have ordered a new injector ($75  ) that will be here tomorrow. I just took the car for a rip around some mountain roads, and took another look at the spark plugs. Again, number four was black. All other three were slightly white on top. I'll let you know what happens tomorrow when I replace the injector.