Engine cuts out under boost

[joshuajayg]

Hey! I'm new to working on Porches but not new to turbocharged cars. I have a Subaru WRX and am learning how to tune on that. I've been a mechanic for six years and am a certified airplane mechanic. Sooo...

I am working on a friend's 1984 930. It has some aftermarket work and was set up for track racing. It has a bigger turbo but we are not sure which turbo it is. It was last worked on in Florida and I live in New Mexico (3,500ft.) The boost builds pretty late as is expected from a large turbo in higher elevations. Once the car hits full boost (.8bar) it starts cutting out hard and won't accelerate well after that. There is a ProComp AFR gauge installed that reads rich all the time, idle to redline. It doesn't have numbers so I don't know the actual AFR and I question its accuracy anyway. I tried adjusting the mixture but that did nothing for the AFR or running condition of the engine.

Where do I begin? I can upload video if that would help.

Joshua

[mark houghton]

Look for air leaks from the turbo to the throttle body (turbo compressor exit piping, intercooler O rings, and any associated hoses attached to the intercooler). I'm presuming this car has the stock CIS fueling system (hasn't been converted over to electronic fuel injection). Your symptoms make me think of air leaks, so confirm good fittings before doing much else.

[joshuajayg]

Alright, Mark. I did a quick visual check and tested using brake cleaner but didn't find anything. I'll be a little more thorough tomorrow. Wouldn't a leak cause a lean condition at idle due to extra air entering the system and not being sensed by the CIS system?

It does have the CIS FI system, BTW.

Joshua

[jsveb]

When my car was way rich under boost, it felt as if it was cutting out. So I would get the AFR verified and not boost until then.

[mark houghton]

Originally Posted by joshuajayg

Alright, Mark. I did a quick visual check and tested using brake cleaner but didn't find anything. I'll be a little more thorough tomorrow. Wouldn't a leak cause a lean condition at idle due to extra air entering the system and not being sensed by the CIS system?

It does have the CIS FI system, BTW.

Joshua

Yes, it would show as lean at idle, but on boost it would be overly rich. At the very least, pull off the IC and check the o'rings. They can split and still seal pretty well until they see boost, then blow out and leak like a sieve.

If all seals are good, then yes it could very well be an issue with your fuel delivery, as in the WUR as the first place to look. First simple check is to see that you have power to the WUR at the plug in. If not, it will stay in warm up mode, being far too rich all the time. Keep in mind that the fuel pumps must be running to check the voltage at the connector...said juice coming via the rear fuel pump relay.

[JFairman]

If it "cuts out hard" maybe the overboost sensor is old and the diaphram has softened up and now it's oversensitive. Anotherwords it might be shutting down the fuel pumps at .8 bar or a hair over that.
They often do that when they get old.

Try grounding the white wire on the overboost sensor and see if it helps.
Over rich mixture usually has a chugging kind of bogged down misfiring response during acceleration with CIS along with stinky black smoke in the exhaust that is easy to see in the rear view mirror.

[Pezho405]

Originally Posted by JFairman

....
Over rich mixture usually has a chugging kind of bogged down misfiring response during acceleration with CIS along with stinky black smoke in the exhaust that is easy to see in the rear view mirror.

What is my problem when this is what happends to me? Sorry im fairly new.
I have a new (Refurbished) WUR, however i think my CO isnt adjusted properly, the car runs rich and that happnds as well.

[Oheggem]

Overboost switch wire maby? Mine did this when the wire was loose. If it cuts off really hard like your about to go through the front window.

[joshuajayg]

Sorry, I guess I should not have said it cuts out real hard. It more of sputters and won't accelerate easily after boost. When I was changing the plugs yesterday I noticed a plug had come lose. It plugs in to the front side of the fuel distributor. After plugging it in, nothing changed.

I will check o-rings here shortly and also the WUR voltage. Where is the WUR? Is there a service manual I can get for this car?

Joshua

[wjfk32]

Overboost sensor was my problem. Make sure its a factory P part.

[joshuajayg]

Just looking at the overboost sensor I couldn't see any part numbers glaring at me. I am assuming it is the sensor with a plug on it with a large hose coming from the intercooler and going out toward the airflow plate.

I am getting 12v to the WUR. The regulator is showing 36 ohms resistance when cold. I'm about to disassemble it and make sure everything is kosher inside.

No splits on the intercooler o-rings and the hoses don't leak either. I tightened up the clamp on the hose from the turbo to the intercooler but that didn't show any positive results.

Joshua

[joshuajayg]

I disassembled the WUR and everything looked really good inside. There was a tiny bit of varnish buildup in the bottom but it wasn't affecting anything. The heating element had a crack in the epoxy but it heated up fine with I applied 12v. I knocked the plug out until it was about flush with the housing on the outside then reassembled it.

Started the car, was running lean. Great! I adjusted the plug in a bit until I was comfortable with the reading on the cheap AFR gauge then reset idle and my mixture screw was now working. Took the car for a drive and it ran MUCH better with just a little missing under boost. After adjusting the mixture again I went for a drive with the owner. The car us just about adjusted. It will now break the tires loose again.

I'm leaving town but when I get back I will install the adjusting screw on the plug, purchase a CIS gauge, possibly a new AFR gauge, and get things set right.

Thanks for your help, guys.

What do you recommend for a gauge? I heard JC Whitney sells one for $60. Is there one available here?

Joshua

[sundevil64]

I was having the same problem of cutting out under boost, but mine was just the opposite.

I put my portable innovate AFR meter on it (couldn't get the other to work) and here is what I found.

At idle warm it was about 12.6. Accelerating in first gear it would richen to 10.3 or so. Accelerating in 2nd it would richen to 9.3-9.4 and it would start cutting out. If I feathered the go pedal to keep the afr above 9.5 it would boost fine (partial boost about .7 bar). If I leaned on the go pedal to attempt to hit full boost it would richen to 9.3 and cut out. I figured I had an air leak. Sure enough I did. The oring from the charge pipe to the IC had torn and blown out. Additionally, I had stripped out the threads on one of the attachment points for the IC. I put in a time-sert, replaced the oring (as I have extra parts on hand...) and bingo! I am at 14.6 afr at warm idle. 1st gear richen to 10.4, 2nd gear 9.7, 3rd 9.3afr with no cutting out just pure power, and 4th is irrelevant.

While I was out running around I kept loosing my coil wire, it would fall off. The sound of loosing a coil wire at full throttle going about 80 really made my ears go up, so to speak.Thankfully, I had a stocked handy dandy tool wrap that allowed me to crimp it and get back rolling. Now I can focus on fixing other issues.

[joshuajayg]

If I give the car partial throttle at higher rpms to decrease airflow, it smooths out for me as well. I will be looking for more air leaks and such next time I'm in town to work on this car. He has an MSD setup for me to install, valve adjustment, adjustable WUR work, and some other small things he wants me to do so I will be getting a bit deeper into things soon. I will install a better AFR gauge too.

So is target AFR under .8 bar somewhere around 9.5 for these cars?

[Tippy]

At .8 bar, I'd shoot for around 12:1 AFR.