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Split second stutter under full throttle

 
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dhillgard  



Joined: 15 Apr 2013
Posts: 13
Location: Edmonton Alberta

PostPosted: Wed Mar 09, 2016 4:13 am    Post subject: Split second stutter under full throttle Reply with quote

Hi all,

1980 924 turbo north american car

I have done a bit of searching around the board for similar problems, but searches related to stuttering, stumbling, running rough often bring up problems that are occurring at idle or just above it, or when the car is cold and the problems are fuel related such as WUR or AAV.

Although my car does run too lean while it is cold, my problem doesn't feel fuel related, and occurs when the car is hot. I will be taking out the WUR to take it apart later today.

I haven't read anything like this, and I'll do my best to describe it as I've only experienced it on 2 drives. (I've only had the car out a few times since I got it running and got it shifting gears again)

The car is hot when this problem occurs, at normal operating temperature. The gauge shows right in the middle, and I do trust it since an infrared heat gun is giving me about 90C on the upper rad hose. We have had unusually warm weather here, so the roads are dry and I took the car out twice in the winter while the temperature was about 0C

Only noticed this problem in 3rd, 4th or 5th gear but mostly in 4th or 5th because they are taller gears I think.

Low speed or RPM drivability is fine, car runs great if i don't have a "lead foot", also starts up and idles fairly well, even on a cold Edmonton day of about 0C.

The problem:
But when I've been driving for a bit and then I put it right to the floor, and the boost picks up through 2500-3000 rpm roughly, It pulls strong until suddenly at a range of say 4500-5500 RPM It suffers a split second of complete power loss, as if I lifted my foot completely and then stomped it back on the floor as quickly as I could, enough to give your head a fairly good rock. (but I didn't lift)

The stutter could happen once as I accelerated through a gear, or could happen multiple times and shake enough that I would ease off to shift to the next gear for the unpredictable (good or bad) behaviour of that gear.

Throughout the drive this did not happen every time I put the car under hard acceleration. It would maybe stutter through one gear, then the next gear would be fine, and The Car would behave normal for 5 minutes of hard driving afterwards, just to start stuttering through a few hard pulls after the next stop light.

The problem seems to get worse as the car is hotter. On a short drive the other day each of the stop lights I pulled away from I did not notice any problems, and I was trying to recreate this. However that was only a 10 minute drive.

I will note the car is making 13psi due to the previous owners messing with the wastage, and does not have an intercooler. I am running premium gas, I would assume if this is detonation the problem would be continuous and not intermittent when under-load

So any thoughts?

Thanks in advance for the advice!
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Rasta Monsta  



Joined: 12 Jul 2006
Posts: 11723
Location: PacNW

PostPosted: Wed Mar 09, 2016 4:33 am    Post subject: Reply with quote

You need to do fuel pressure tests and pump volume test per FSM.

Lean running is the absolute worst thing for your 931, hands down.
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  • WeiBe (1987 924S 2.5t) - 931 S3
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dhillgard  



Joined: 15 Apr 2013
Posts: 13
Location: Edmonton Alberta

PostPosted: Thu Mar 10, 2016 6:14 am    Post subject: Reply with quote

Well yesterday I was able to pull of the WUR and have a look inside of it, possibly the cleanest one I've ever seen. I've pulled them apart before and have had them fall out in a bunch of small rusted pieces, but this one looks good as new. So while putting it back on I also solved my (noticeable) running lean issue.

The car was not running noticeably lean on the first drives I took the car on and noticed my stutter problem.

on more recent drives I noticed the lean running. When I say noticeably lean, I mean the car was running rough at cold idle, and was not very responsive to me opening the throttle up slowly, as if adding more air was not doing it any favours. But this was my own fault it turns out.

Recently I had the heater core/box out to get the blower motor unceased, and I lost the metal clip for the WUR electrical connector, which I removed to fit my hands down there. When putting the car all back together, I stole one of those metal clips from the electrical connector I have circled in red in the below picture. This was since I though to myself I will remember to get another later, but didn't! Not sure what that piece is but it is inline with the smaller fuel line to the WUR. After getting another metal clip from my parts car and plugging that connector back in the car starts/idles/runs below 4000rpm absolutely great while its cold or warm or hot now. as long as i don't take it to High RPM, as even though I have plugged this connector back in, the intermittent stutter persists.

Is there sensors somewhere that could cut spark to these engines for some reason? The car really does fall absolutely flat on it's face for 1/2 second, and then puts everything it has back to the wheels instantly.

That all said, testing of the fuel system is on my to do list once I finish researching what that all includes!


The piece I left unplugged for a drive, however since I have plugged it back in the car runs better cold and at idle, but this stutter remains (not my car, image edited from google)

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1978 924 w/ 79 engine - White (Daily driver)
1979 924 - Red (Stripped for parts)
1980 931 - Petrol Blue (Looks green to me!)
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Rasta Monsta  



Joined: 12 Jul 2006
Posts: 11723
Location: PacNW

PostPosted: Thu Mar 10, 2016 6:32 am    Post subject: Reply with quote

The overboost switch kills the fuel pump if ground is broken. It is sticking out of the charge tube to the left of your square in the picture. Worth rewiring is the connection looks crusty.
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dhillgard  



Joined: 15 Apr 2013
Posts: 13
Location: Edmonton Alberta

PostPosted: Thu Mar 17, 2016 5:39 am    Post subject: Reply with quote

Problems fixed, and understood (I think)

Does anyone know what pressure the overboost switch kicks in? 1 bar? On the long hard pulls I paid attention to my boost gauge and if it creeped up to 15 psi the engine would cut out and then power would come back at around 11-12psi.

Recently, the pressure line from the charge tube broke right off at the fitting on the wastage. After this occurred, the car would only make 10psi and you could hear the leak very obviously. I knew something was wrong so I tracked down the broken line after that drive.

I have since temporarily repaired the pressure line by slipping 5/16'' fuel line over it and clamping it on. I then took the car for a drive and was at first surprised to see it was only making 5-6 psi!

This has had a drastic affect on the cars acceleration haha!

I'm guessing the line had been cracked near the wastage the whole time (until it broke) and was bleeding off the control pressure keeping the wastage closed for longer and this was allowing the car to make the 13psi I mistook for "normal".


picture 1: where the break occurred
picture 2: previous owner really didn't want that pipe to fall off...



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1978 924 w/ 79 engine - White (Daily driver)
1979 924 - Red (Stripped for parts)
1980 931 - Petrol Blue (Looks green to me!)
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ideola  



Joined: 01 Oct 2004
Posts: 15548
Location: Spring Lake MI

PostPosted: Thu Mar 17, 2016 6:02 am    Post subject: Reply with quote

Is the bracket to your lower charge tube intact?
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dhillgard  



Joined: 15 Apr 2013
Posts: 13
Location: Edmonton Alberta

PostPosted: Fri Mar 18, 2016 3:16 am    Post subject: Reply with quote

I will be able to get under the car and have a look this weekend, what makes you ask Ideola?
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1978 924 w/ 79 engine - White (Daily driver)
1979 924 - Red (Stripped for parts)
1980 931 - Petrol Blue (Looks green to me!)
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Rasta Monsta  



Joined: 12 Jul 2006
Posts: 11723
Location: PacNW

PostPosted: Fri Mar 18, 2016 3:30 am    Post subject: Reply with quote

What that WG can look like:


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ideola  



Joined: 01 Oct 2004
Posts: 15548
Location: Spring Lake MI

PostPosted: Fri Mar 18, 2016 3:37 am    Post subject: Reply with quote

The lower charge tube is just an interference fit on top of the turbo outlet. There is a small L-bracket that fastens the lower charge tube to the head. It frequently goes missing. When it does, boost pressure WILL cause the entire charge tube assembly to lift off of the turbo outlet, causing a "fluttering" loss of boost. Given the condition of other items in the boost tract, it would not surprise me if this were also occurring.

With these cars, you will learn that there are often multiple issues resulting in the symptoms. Don't assume that one fix will solve the whole problem.
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Power Tryp  



Joined: 16 Apr 2009
Posts: 434
Location: Calgary, Alberta

PostPosted: Fri Mar 18, 2016 3:42 am    Post subject: Reply with quote

Rasta Monsta wrote:
What that WG can look like:



Gentlemen, look at your car, now back at me, now back at your car, and back to me. Sadly this is not your wastegate, but this is the wastegate your wastegate could look like. I'm on a Porsche.
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NoName - 1980 931 (seeking rebuild)

Green tartan interior, you don't see that on a Civic.
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Ian89C4  



Joined: 01 Apr 2011
Posts: 561
Location: North Carolina

PostPosted: Fri Mar 18, 2016 6:56 am    Post subject: Reply with quote

Nice
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Ian Edgerly
North Carolina

Current:
1981 924 SCCA/ChampCar Weissach Race Car ("Serenity")
1987 924S ChampCar Endurance Car ("Der Weg")
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dhillgard  



Joined: 15 Apr 2013
Posts: 13
Location: Edmonton Alberta

PostPosted: Tue Apr 05, 2016 6:25 am    Post subject: Reply with quote

Have had a chance to look at that charge tube now, and the bracket is still there, car seems to not have any major boost leaks currently, but the rubber hose before the throttle body is ready to give up!

Next step is a compression test, a lot of oil in the air filter housing.
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1978 924 w/ 79 engine - White (Daily driver)
1979 924 - Red (Stripped for parts)
1980 931 - Petrol Blue (Looks green to me!)
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