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Bp4W Vacuum Leak Chasing.

vacuum leak bp4w base idle

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#1 hypno_chode

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Posted 08 May 2020 - 04:02 PM

Hi folks,

 

Ive been battling an issue with my Mk2 (BP4W engine) on and off since I purchased the car about a year an a half ago. The car would drop to around 500 rpm when coming to a stop e.g. at a junction or slowing down to a crawl while on the brakes. I did the obvious thing and clean the IACV and set the base idle, although I quickly found that I could not get the idle to go below 950-1000rpm even when the screw was all the way in and driving the car like this was just as bad as before if not worse. The only way to make the car more drivable was to back the screw off enough that the revs wouldn't drop but this meant that the base idle was far too high, around 1400 rpm.

 

This seems to be a vacuum issue but I feel like I've been round pretty much everything on the intake side, list follows:

 

Replaced all vacuum lines including port caps.

Vacuum tested the brake booster and check valve.

Vacuum tested the VICS valve and check valve.

Cleaned manifold including all the ports and refitted with new OEM gaskets and injector seals.

Checked that the mating surfaces on the inlet manifold were flat using a proper straight edge, not just an engineers rule.

Cleaned EGR valve, checked that it was seating properly and replaced the boost solenoid.

Tested the TPS.

Changed the IACV to a new aftermarket unit.

Swapped the complete throttle body, TPS and IACV from a working Mk2.5 vvt (which uses the same Throttle body, TPS and IACV), I even put my throttle body onto the later Mk2.5 engine and managed to set base idle, no problem.

Ive also tried changing the MAF and a new coolant temp sensor although I was doubtful it would fix the issue.

 

After all this I am still unable to properly set the base idle and I have to back the screw off enough to stop the revs dipping as previously mentioned. Is there anything else Ive missed that would be worth checking and is there a possibility it could be an electrical fault causing a component to open when its not supposed to or am I on a wild goose chase?

 

Any help or advice would be greatly appreciated.

 

Cheers,

Muir

 

 

 


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#2 dadbif

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Posted 08 May 2020 - 09:32 PM

Ignition timing?
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#3 carl s

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Posted 09 May 2020 - 09:05 AM

Silly question but have you checked the engine earths?


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mk1 1991 Eunos with RS NB drivetrain: https://www.mx5nutz....howtopic=322586

ex - mk3 2006 2.0 sport

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#4 hypno_chode

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Posted 09 May 2020 - 04:27 PM

Thanks for the suggestions so far folks. Ive had another look at the earths and given them a once over. I had to replace my rad today because my old one didn't want to retain water anymore hahaa. Ill check the timing tomorrow when Ive got the chance and see if there's anything out of spec or if the earth clean ups has made a difference.


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#5 hypno_chode

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Posted 10 May 2020 - 02:28 PM

Update: Failed to get a reading for the ignition timing as I couldn't get my timing light to work with the car but worked when tested on another vehicle. Which suggests a bad earth? I tried a direct feed form the battery as well as the terminal on the alternator, the light wouldn't give a consistent flash or any flash at all. Ive checked the earths as follows; underneath headlight, front of inlet manifold, back of the engine that attaches to the bulkhead, wiper motor, rear of PPF, pretty sure I found one under the inlet manifold when I had it off. Id imagine there's probably more? The only other electrical fault Ive noticed is the wipers would slow down once in a blue moon, could this be related? Gonna go on an earth hunt and see what I find.


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#6 carl s

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Posted 10 May 2020 - 10:38 PM

If you have a test meter check for continuity on the engine parts, the head, inlet manifold, block, throttle body, all have gaskets separating them. I'd then check the resistance between each component and the same earth point on the chassis (a proper earth point or bolt head etc.). When I installed an NB motor in my NA I found lots of earths all over the place at the back of the engine & inlet manifold.


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mk1 1991 Eunos with RS NB drivetrain: https://www.mx5nutz....howtopic=322586

ex - mk3 2006 2.0 sport

ex - mk2 1999 1.8is JRSC


#7 hypno_chode

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Posted 11 May 2020 - 07:18 PM

Thanks again Carl, I'll run some tests in the next few days. In regards to the ignition timing, I borrowed another timing light an it works great but I realised I wouldn't get an accurate reading if I cant set my base idle, which is why I didn't try checking it sooner.


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#8 hypno_chode

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Posted 20 May 2020 - 02:43 PM

One thing I forgot to mention is that on the original throttle body I noticed that someone prior to my ownership had opened the throttle stop, probably in an attempt to fix the rev drop. About a month ago I had the battery disconnected (and drained any residual current) for a few hours and closed down the flap and made sure it didn't stick. After that I let the car warm up and I was able to set the idle fine, took it a test drive, no rev drop. I thought it was fixed then two days later it was back to its usual ways, would this still suggest an earth issue? Ive done some continuity tests like Carl mentioned although I couldn't find anything conclusive. Still need to recheck the earths under the headlights although they have been cleaned since Ive owned the car.


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#9 carl s

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Posted 20 May 2020 - 06:28 PM

On the NA there is a procedure that involves disconnecting the battery amongst other things to reset the idle, not sure on the NB sorry. That might have some bearing on you disconnecting the battery earlier.


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mk1 1991 Eunos with RS NB drivetrain: https://www.mx5nutz....howtopic=322586

ex - mk3 2006 2.0 sport

ex - mk2 1999 1.8is JRSC





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