Sudden & Abrupt Boost Loss
 

I am experiencing a sudden cut out of boost, it has happened a couple times in the last couple years. Was never able to reproduce. Yesterday on a 300 mile drive to a car show I am getting it continuously with some other weird stuff going on.

Here are the symptoms
Factory Boost gauge ( Know it inaccurate) is flopping back and forth from 0 bar to full deflection even prior to producing typically much boost under 3000 RPM.

When I open the throttle quickly, it starts to build boost and then cuts out like the fuel or ignition is cut. Over boost protection I am guessing?

When I open the throttle slowly and gradually the boost builds slowly, and the car accelerates slowly with the boost needle flopping like a fish. If I don’t let it build slowly and open the throttle to quickly it cuts out until I retard the throttle. I also had the car not want to start after a rest stop break after sitting hot for 10 minutes (it has never done that before) this may or may not be linked. I have a spare yellow boost relay and was going to try that when it finally started. Had to crank it 6 times and it fired right away.

I have read through most of the old threads on this and have some guesses of what it could be, what I am looking for is systematic approach to tracking this problem down. Best guess is electronic then mechanical easy to complex. Any one have a flow chart of where to start based on these symptoms. I am looking for a logical trouble shooting game plan.


87 US car
Mod's include Turbo (k27 Hi flow) complete B&B exhaust system/headers, Kokeln IC, SC Cams, AI flowed fuel head, air pump and cat removed, 1 bar boost spring in stock waste gate. 76,000 Miles

I had symptoms very similar to yours.... The problem was intermittent and difficult to diagnose.... I carried a test CDI box, a couple of fuel relays, an alligator clip to jump the fuse, and the dreaded overboost relay... When it crapped out, I started replacing things... It was my Permatune that was bad... Rebuilt Bosch box and I was good to go... :)

Mine was doing that and the issue was I forgot to tighten the ground wire for the cut-off switch where it screws to the i/c. Had it fallen off I would have been walking home :(

I have checked the yellow boost relay no visible problems with the wiring or fuses. Anything else wiring or relay related I should look at? Before going to the Blow off valve and waste gate.

Sometimes the written word doesn't adequately describe the symptoms. By "cutting out", do you mean stuttering/misfiring/loss of power, or do you mean "dash in the face" complete and radical immediate engine shutdown (as when the overboost circuit kicks in)? If not the latter, then I would first look for boost leaks.

Try this... ground your overboost switch that is screwed into your BOV. If car runs normal then your switch is bad. I had this happen to me at Roebling Road... luckily not in the middle of a corner but on the straight. Grounded the overboost switch wire to some random nut and finished the DE.
Then, try this... open up the fuse/relay panel in the front luggage compartment. If either of your two relays (the two big red ones nearest the windshield) are falling out of their sockets, push them back in and enjoy your day. Please don't ask me how I know this as it was a long trip home :)
The symptoms you describe sound like the fuel pumps are cutting off. The easiest way to check this is turn the key to run and push down on the plunger. You should be able to hear both pumps running with the key turned to run, but the engine not started. One pump is right there above the driver side rear tire. The other pump is in the center front of the car tucked under the sway bar. You might need someone to push on plunger while you go towards front of car. They are amazingly loud though without the engine running to drown them out.
Before you buy an expensive replacement pump, change the much cheaper relays.
Not a flow chart but here is a check list of sorts...
1. Ground overboost switch, see if that fixes problem (if so, then replace overboost switch - cheap)
2. Make sure relays are seated (if not, push them back in - free)
3. See if both fuel pumps are running (if not, replace relays or swap them to determine bad relay and replace - cheap)
4. Bad fuel pump (if so, R&R very expensive)
5. If all that crap is working come back and let us know. :)

Yes immediate like the fuel is cut.

Like the over boost is kicking in very early in the power band.

Thanks DSP I will approach in that order and let you know.

DSP has provided a good troubleshooting list for you. The only thing I would add is that in checking your fuel pump operation, there is a better way than pushing down on the airflow metering plate. Pushing down the metering plate will dump fuel out the injectors into your cylinders. If you reach around the back of the fuel head, there is an electrical plug. Unplug it and the pumps will run with the key in the "run" position. When you're done troubleshooting, just plug it back in. Here's a photo of the plug I'm referring to (circled in green), although its an SC, the location of the plug is similar. I can get to it on my car with the factory airbox in place. http://forums.pelicanparts.com/uploa...1251232374.jpg

True dat Jwasbury. It's a b.i. itch to reach around there though sometimes. That is how I check mine but without a picture it is hard to explain where that plug is located. The plunger trick is usually easiest for folks to understand. LOL

My money is on the overboost sensor/switch. Unplug and ground the single wire.

The switch is normallly closed to complete the ground connection, and is supposed to open and interrupt the ground circuit at something like 1.2-1.4 bar if memory serves me. Keep in mind that if the wire to the switch has a break in it, it can shut you down intermittently. Reach back there with the engine running and wiggle/tug it first to see if your engine stops. It's all a process of ellimination.

It seems like your overboost switch is tripping. It might do so beacuse of bad electrical connection or beacuse engine is actually overboosting.

If cleaning the connector and checking the relays don't solve the problem, try this:

Check wastegate membrane by blowing air trough top vent pipe (one going from top of the wastegate vent, exposed to air). If air leaks trough it, your wastegate membrane is toast and car is overboosting.

If membrane is tight, you can do this:

Ground overboost switch and carefully coax the car into boost in 4:th gear while observing boost pressure with verified gauge. If gauge goes past 1.0 bar (don't push it further than 1.1 bar) you have overboost condition due to spring being too hard or wastegate valve is stuck.

I had a problem like that. Drive you nuts. Everytime you go to look for it, it runs away.
I solved mine by hooking a meter on my overboost switch. The terminal was a bit loose internally, and every now and then flopped around inside. I think I hooked some air pressure on to the switch (14 psi) and wiggled the connector and watched multi meter.
I could make the car stop by wiggling the spade terminal. At that point I felt happy - problem solved.
Alan

The overboost switch is there to protect you. Do make sure it works correctly but also make sure the Wast Gate is working right.

Back after your suggestions

Thanks all

1. Ground overboost switch, see if that fixes problem (if so, then replace overboost switch - cheap) Going to wait on this one

2. Make sure relays are seated (if not, push them back in - free) OK Fine

3. See if both fuel pumps are running (if not, replace relays or swap them to determine bad relay and replace - cheap) Ok Fine

4. Bad fuel pump (if so, R&R very expensive) Think I dodged the bullet
5. If all that crap is working come back and let us know. SmileWavy

I'm back, drove the car getting the same abrupt cut out. I went to the next step I wanted to make sure the waste gate was working prior to the switch grounding procedure. Waste gate held a light compressed air charge in the top hole. I took it apart anyway to make sure nothing was getting hung up. I will clean and put it back on looks fine to me.

I will ground the switch and try the test when I know the WG is working

Question: The metal tubular line off the top of the waste gate just goes up throught the engine tin and terminates with nothing on it?


I found this old thread describing the cut as dash in your face same exact situation as this. Boost gauge needle is still erratic. Boost starts to build and about 3,500RPM dash in your face until you back off the throttle.


http://forums.pelicanparts.com/911-930-turbo-super-charging-forum/474194-930-cutting-out.html

Thanks for the help ! What next ?

The top outlet on WG goes to vent/waste. It is just a vent to let the diaphragm expand with no back pressure.
You can check WG in place basically - disconnect boost hose from IC, and pressurise from air bottle, compressor, whatever. If possible hook a Press gauge in the line and you will hear it open, and see the opening pressure on the gauge.
You dont even have to remove unit.
When car is idling, try wiggling the spade terminal on your overboost switch - see if can make it die.
You can also hook o/boost switch to pressure gauge and slowly pressurise, with multi meter on terminal and grounded from memory. It is just a switch, but should not operate before 1.1 bar. I think I just stuck a piece of plastic tube over the sensor end, with a gauge/tee piece in between and hooked air on it. To check my new one.
You may find it has gone a bit soft and triggering at 0.8 or anyplace inbetween. But if you do this test you can repeat as often as want til satisfied is 100% reliable.
Alan

The erratic boost gauge is telling us something. Either a faulty boost sensor unit (hope not, they're about $150) but since it seems to be tied to your problem, it's almost pointing to the WG. But then, if your boost gauge is doing the chicken even when not on boost, then the sender or gauge or connections could be the simple culprit to that symptom...regardless of the other engine running symptoms. You may have two separate issue going on here.

So lets for now forget the funky gauge bouncing. Do pull the boost cut-out switch and put some external pressure to it. It should open and interrupt ground at around 1.1 to 1.2 bar I believe. If it opens much sooner than that (like way before .8 bar) then you've found your problem.

And do check the WG operation as already mentioned. Leave the top port vented to atmosphere, and connect a pressurized air source to the bottom port. Stuff a wad of paper in the WG exhaust pipe, and with the car running (don't forget to block the now open line to the intercooler) your wastgate should open at anywhere between 15 to 18 psi...and blow that wad of paper out. You'll hear the exhast note when it opens.

Mark Thanks

Waste gate checks out, opened at 14 pound/square inch = 0.965 bar which is good it's a 1.0 bar spring.

I will pressurerize the the boost safety switch in a couple days and report back on my findings.

Thanks again Pelicans !!!

My money is on the o/boost switch. Same issue I had. Have you tried wiggling the terminal when idling? Do that first, in case it is a bad terminal rather than faulty sensor.
Alan

Ground the wire going to the overboost switch. That is the only thing that can shut down the fuel pumps if all your relays are in good working order and both pumps are running.

I'm back, took the car out this morning with the over boost safety switch grounded. Several boosting sessions, waste gate wailing.

Car ran fine, no cut out strong boost as before. Boost gage needle was more stable as well.

I think I found the problem! need a new switch! this one is tripping .3-.5 Bar

This is an easy fix, ordering one right now from Pelican.


Thanks all for the help figuring this out.