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16Volt 16Volt is offline
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Join Date: Oct 2013
Location: Seattle, WA
Posts: 987
Cruise surge/boost issues

About 9hrs after signing the paper work on the car it developed and issue with cutting out under boost occasionally, down on power, stuggling to rev etc. I had John do a full tune to get us to a good baseline.

So post tune up (plugs, cap/rotor, valve adjustment) I took the car for a jaunt on some back roads late in the evening. Car pulled hard and clean, no more cutouts under boost, no more feeling like it struggles to build rpm. Sweet!

There is now some lifter tic on the passenger side bank but I don’t have any point of reference as to what is normal valve noise, nothing alarming however. John said most the intake valves were way too tight and one cylinder needed the exhaust valve tightened. The car does feel like it may be just a hair down on power but I am fine with it given it makes power smoothly and cleanly.

A few days later I look the car for a longer cruise and noticed that once the car is warm and you are holding steady at 2100-2600rpm the car surges. It feel like someone is just waving their foot over the brake. Not a misfire, but a momentary hesitation, the tach doesn’t register any drop in rpm but your head just bobs forward and back. Get on the power, no issue. Lift completely, no issue. Hold it at a steady rpm, head bobbing. Argh. Over the next week or so much more road testing. Issue is consistent and ever present once the car is warmed up. Around town you don’t notice since you are always accelerating and decelerating, on the highway however it is very noticeable and after about 10mins super aggravating. Called John and setup an appt.

The car when I bought it had the 02 sensor disconnected, post tune up it was left unplugged. I have heard from a few different folks that this is not all that uncommon (true)? John plugged it in to troubleshoot and found it was completely dead anyway. He tossed a used unit on and dialed in the cold and warm mixtures. After a road test the car was ready and I picked it up. Unfortunately I only made it about 10 blocks. The car had trouble accelerating in any gear and was really having trouble revving. Swung back to shop and we disconnected the 02 sensor and reset all the mixtures again. Car went back to its normal self, phew. I took it out later that night, the surging remained. Bummer. John did relate to me that all 930s exhibited some surge at cruise, and I thought perhaps the cams could be exasperating the issue. I agreed to put some more miles on it and see what happened. I put another 500 miles on the car and no change. Car was great in every scenario except cruising.

Skip to Friday, the car has been great just as long you never want to keep it at one speed. I popped the engine lid when I got from home to work to take a quick peak. I like to keep an eye on “new to me” engines and honestly I am kind of at a point where I think it’s time to take a little more mechanical ownership. One of the vac lines looked janky to me, the line running to what I believe is the advance side of the distributor vacuum canister. I reached in to snug it up on the nipple and low and behold, this is what I found.

IMG_5678.jpg by thecrashingdoor, on Flickr

IMG_5680.jpg by thecrashingdoor, on Flickr

Yep, a giant hole. The nipple was basically poking clean through. I cut the line back and put it back on the nipple with a ziptie to secure it. Drove the car and…..all the initial pre-tune up issues are right back. Cut out and struggling under boost arghh. The surging remains but feels slightly less severe. So here we are, back at square one.

So here is my new theory. The root cause of the boost cutouts, surging and struggling to build power are all ignition related. When I think of the problems in terms of advance and retard it begins to make sense. The cruise surge could certainly be someone cranking advance and alternately yanking it back out. The struggling to make power/build rpm under boost could also certainly be excessive retard. I have been researching the 930 ignition system and the vacuum diaphragm/distributor while not a common problem could be suspect. Old grease can cause the weights in the distributor to mess with timing. Vacuum diaphragm can deteriorate and mess with timing etc.

If I disconnect the line from the distributor to test is there any danger? Essentially I think this is how the car has been running with the giant hole in the line.

I have a new front fuel pump, new rebuilt WUR and a adjustable WUR on the way. I am going to swap the pump just as preventative maintenance since its a little noisy. Its going to be awhile till I can get the WB 02 installed still.

Any suggestions folks?
- 86 Porsche 930 - Mr.Hyde
- 86 BMW 635CSi - Dr.Jeykell
- 2006 Infiniti M45 Sport
Old 02-17-2014, 01:10 PM
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