Cutting out on full boost

[mark houghton]

I'm informed enough to "think" I know what's going on...I think...but always appreciate feedback.
Last spring after doing all sorts of stuff - like valve adjustments (a PITA) and replacing a CV joint (a mega PITA), I also removed the belt to the smog air pump, got rid of the mail box and went with a K&N filter, and capped all the assorted breather hoses with filters. Pretty standard stuff.

Now on a WOT 3rd gear pull, I get to around 100 mph on full boost and get a brief 1/2 second hiccup. Not complete shutdown/face in dash as one would expect if the overboost circuit was shutting off the pumps, but yet I suspect that's what's happening just momentarily. Boost gauge never exceeds .8 bar and usually holds at around .7 bar as these are often inaccurate anyway.

So I'm thinking my OB switch is becoming problematic. Next time out I think I'll pull the wire to it and ground it...see what happens.

[gsxrken]

Repacking the CV joints IS a nasty job. Agree with you there.
What’s your AFR when it hiccups?
Grounding the wire is the logical step but if it still does it, I’d suspect a weak fuel pump. Same thing kicked my tail for half a season a few years ago. I had to splice a pressure gauge in between the two fuel pumps (covered in the shop manual) to determine which pump wasn’t carrying its weight.

[mark houghton]

Originally Posted by gsxrken

Repacking the CV joints IS a nasty job. Agree with you there.
What’s your AFR when it hiccups?
Grounding the wire is the logical step but if it still does it, I’d suspect a weak fuel pump. Same thing kicked my tail for half a season a few years ago. I had to splice a pressure gauge in between the two fuel pumps (covered in the shop manual) to determine which pump wasn’t carrying its weight.

No idea what my AFR's are...I no longer have a gauge as I did on my past 930.
Weak fuel pump could be possible though one would think it would result in a more sustained stutter from fuel starvation. But as soon as it hiccups I let off the gas so I don't know if it would continue.
First things first...I'll pull the OB wire and ground it.

[carrpet]

Hi Mark
I have experienced similar issues in the past. I would start with the overboost switch from my experience over the decades was opening at lower boost levels. My OB switch is now shorted out. Over-boost is now managed by microsquirt which tells my ignition to retard above a safe threshold level of say 0.9bar. Cutting out fuel pumps at WOT is not good news anyway which must result in overlean AFRs.
I tested my OB switch using a compressor and pressure regulator. I actually bought a new OB switch from Porsche and found it opened at an even lower pressure and sent it back.
I also removed my centrifugal rpm limit switch in the rotor arm which also proved to be suspect and was cutting out at prematurely at low RPM. RPM limiting is now managed by my programmable Classic Retrofit CDI+ ignition
Peter

[Paullparts]

Originally Posted by mark houghton

I'm informed enough to "think" I know what's going on...I think...but always appreciate feedback.
Last spring after doing all sorts of stuff - like valve adjustments (a PITA) and replacing a CV joint (a mega PITA), I also removed the belt to the smog air pump, got rid of the mail box and went with a K&N filter, and capped all the assorted breather hoses with filters. Pretty standard stuff.

Now on a WOT 3rd gear pull, I get to around 100 mph on full boost and get a brief 1/2 second hiccup. Not complete shutdown/face in dash as one would expect if the overboost circuit was shutting off the pumps, but yet I suspect that's what's happening just momentarily. Boost gauge never exceeds .8 bar and usually holds at around .7 bar as these are often inaccurate anyway.

So I'm thinking my OB switch is becoming problematic. Next time out I think I'll pull the wire to it and ground it...see what happens.

Hey Mark,

That quick hiccup sounds like the overboost switch is right on the edge of tripping. Grounding the wire to test is a good idea, just watch the boost so you don’t hit real overboost. If it stops happening, you’ll know the switch is getting weak and can replace it. Might also be worth checking your gauge with a mechanical one since they can read a bit low.

[patina]

Originally Posted by gsxrken

Repacking the CV joints IS a nasty job. Agree with you there.
What’s your AFR when it hiccups?
Grounding the wire is the logical step but if it still does it, I’d suspect a weak fuel pump. Same thing kicked my tail for half a season a few years ago. I had to splice a pressure gauge in between the two fuel pumps (covered in the shop manual) to determine which pump wasn’t carrying its weight.

Same exact thing I went through. Had a hell of a time figuring it out because the car passed the flow test easily. Ended up replacing my supply pump with a -044 and problem solved.

[mark houghton]

UPDATE:
Grounded out the OB switch wire today and did a short "spirited" blitz, 3rd gear WOT pull from 60 to about 100, no hiccup. Need to repeat a couple more times in a less congested area. Fingers crossed that that's the issue.

[gorskined]

Originally Posted by mark houghton

UPDATE:
Grounded out the OB switch wire today and did a short "spirited" blitz, 3rd gear WOT pull from 60 to about 100, no hiccup. Need to repeat a couple more times in a less congested area. Fingers crossed that that's the issue.

fingers crossed its the switch and you are not actually over boosting . I've never experienced it but a small leak in the waste gate or a loose connection can slow down the waste gate just enough and you cant see it on your boost gauge. This will cause you to over boost and trip the over pressure switch. if that's happening by removing the over boost switch for testing that could open the door to some real issues during the testing . i personally would just remove the switch connect a mity-vac to the switch and see what pressure it trips at .

[mark houghton]

Originally Posted by gorskined

fingers crossed its the switch and you are not actually over boosting . I've never experienced it but a small leak in the waste gate or a loose connection can slow down the waste gate just enough and you cant see it on your boost gauge. This will cause you to over boost and trip the over pressure switch. if that's happening by removing the over boost switch for testing that could open the door to some real issues during the testing . i personally would just remove the switch connect a mity-vac to the switch and see what pressure it trips at .

All good points. It doesn't feel like it's overboosting, whether or not the gauge is accurate. I fact it doesn't feel quite as strong as it should at .8 bar but then again the old 3LDZ's start losing their poop past 5500 rpms.

On my past 930 it pulled all the way to the top with a 7006 turbo, so I really can't compare the two. Only once did I experience (on that car) an overboost situation and when that switch trips its catastrophic complete shutdown. Not so with my current situation, more like a very short miss or hesitation. No death rattle from pre-ignition, thank God.

I did pull the switch yesterday and hooked up to my cheap Chinese "Mity-Vac" but couldn't apply much more than 1.0 bar for some reason...maybe 1.2 momentarily. The switch held...they're supposed to trip at 1.4 when new.

I'll check the WG hose and connection. I did pull it off when doing the valves this spring

Could be I'm over rich on boost and maybe flaming out a plug or two for a second (brand new Iridiums). Need to purchase and install an AFR

[mark houghton]

Just thought I would bring closure to this thread I started. The last few weeks I've been running with the OB switch bypassed/grounded. No issues with any split second miss when at full boogie.
So I removed the switch and hosed out the internals with electronic contact cleaner, reinstalled and hooked back up. Took a spin today, lugging in 3rd at 60 followed by a full pedal sustained stomp cranking .8 bar up to the century mark and all is good. Same result if in 2nd and backing off at 85.

Just something to think about, vs $80 for a new switch.

For what it's worth, sure enjoyed a nice early autumn romp.

[SLNT930]

This has started happening to me. Complete shutdown. NOT the face in the dash feeling... At all. Car shuts down and continues to coast. Am able to restart while moving. Will start with checking and cleaning my switch.

[mark houghton]

Originally Posted by SLNT930

This has started happening to me. Complete shutdown. NOT the face in the dash feeling... At all. Car shuts down and continues to coast. Am able to restart while moving. Will start with checking and cleaning my switch.

If the OB switch is suspected, the first diagnostic is to unplug the wire to it and ground it to earth. That will take the switch out of the equation.

[SLNT930]

Originally Posted by mark houghton

If the OB switch is suspected, the first diagnostic is to unplug the wire to it and ground it to earth. That will take the switch out of the equation.

Yes and I highly suspect the switch. I have 100% of the service records and it's original to the car. 38 years old now.

[sunlion]

Funny Mark…check your Facebook messages to me from a couple years ago. Lol. My distributor shaft bushing was out of round and there was a lol of wobble. Did all the usual fuel flow checks and grounding/replacing the OB switch but still got the hiccup on a string 3rd gear pull. Finally tuned it with an afr gauge and it’s gone (so are my burbles…need to dial those back in) but haven’t really driven to feel confident in the fix. (Working on the lighting system electrical to solve recurrent blown rear taillight fuse problem.)