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930 engine problems..please help

Ok guys... I am not new to porsche cars at all, and pretty good backyard mechanic, but this one has me stumppppped.

My 84 turbo when under load misses out and pours black smoke... most of the time. It is cis injected of course and I am pretty familiar with it... had lots of 911sc's, 928's, etc... vw's etc... you get the idea.

I have check, I BELIEVE, all the vacuum leak posibilities, checked the airbox for being broken, changed plugs, played with fuel mixture, everything under the sun that was obvious to me. When it is really cold out, hence right now, it will run down the road and the turbo will kick in and run ok... not perfect, but not pouring smoke. When its warm out, such as noon today, it barely stumbled through town with out turbo and would look like a diesel with the turbo kicked it.

It was suggested that a fuel pump may be out... I was told it has two. Thoughts?

Also, it will rev up just fine without a load when it is acting up... no smoke, etc... just not in gear or with a load behind it.

It starts perfect, idles perfect, no other issues.

I had a cis 911 onetime that had a stopped up fuel filter that would make the car rev fine without a load but crap out with a load... Any thoughts there?

But, under load, I get black smoke... i believe is fuel (not oil) and that leads me to believe it is getting fuel...

My tach is acting up too... but not in series with the engine issues... I though maybe spark was cutting out, but that doesnt make much sense because it will always free rev without a load... the permatune box doesnt care if there is a load or not... it doesnt know. So i think spark is ok and I have a fault tach wire or something... thats for a nother day.

i am lost... I am game for any ideas... fuel pump? Sensors? (i unplugged the 02 sensor... but it isn used in full throttle anyway... and doesnt make any difference in my issues now).

Low fuel pressure? Cloged filter? Un obvious vacuum leak under pressure?

Please help.....


todd
Old 10-27-2007, 05:02 PM
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My 2c

The tacho acting up is a symptom of a bad Voltage Regulator, you can check this by measuring the voltage across the battery with the engine running. If the VR has failed you will be overvolting (Should read 13.5-14.5v).

Under load you are pumping a lot of fuel into the cylinders and it requires a strong spark to ignite it. It is possible that overvolting your permatune under load is the cause.
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Old 10-27-2007, 06:01 PM
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Time for an alternator. I'm betting you change that and life will be good.

Yasin
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Old 10-28-2007, 05:10 AM
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very logical... soI tested the voltage and it is 13.7 pretty religeously at all rpm... had a slight drop to 13.5 at idle....

apreciate the thought.... any one else?
Old 10-28-2007, 05:11 AM
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Quote:
When it is really cold out, hence right now, it will run down the road and the turbo will kick in and run ok... not perfect, but not pouring smoke. When its warm out, such as noon today, it barely stumbled through town with out turbo and would look like a diesel with the turbo kicked it.
That sounds like a WUR issue. Do you have a way to measure your fuel pressure under boost?
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Old 10-28-2007, 06:02 AM
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Wink

ok.... wet back out... fuel pressures appear good, though cant rally tell under boost.... guess I could extend my fuel lines, etc... but wait...!!! I tested one more time the alternator, (more awake that at 6am...) and it was fine.... until I got about to 3500rpm.... at 3500 it jumps to 18volts....

I am guess this is my culprit?? Does it have a voltage regualtor in it that i ca swap? I have a box of odd regulators from 928's and various sc's.... I havent pulled the alternator yet.

That cuase bad spark under load?

thanks a ton guys.
Old 10-28-2007, 06:21 AM
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Sorry about the typing above.... wow... dang laptop keyboards and dirty hands...


I really can spell... usually.... ha!!
Old 10-28-2007, 06:22 AM
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The VR is a $20 part, its a PITA the replace. There is a good writeup in the tech section I believe
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Old 10-28-2007, 01:38 PM
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So if the VR is overcharging at 3500rpm and you get on boost the spark can't take care of the extra fuel.
How does that relate to the temperature? The symptom is less severe when cold out, the air is more dense, so the A/F ratio is effectively raised enough to reduce the affect? That must be it. Interesting ....
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Old 10-28-2007, 01:50 PM
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Quote:
Originally Posted by RarlyL8 View Post
So if the VR is overcharging at 3500rpm and you get on boost the spark can't take care of the extra fuel.
How does that relate to the temperature? The symptom is less severe when cold out, the air is more dense, so the A/F ratio is effectively raised enough to reduce the affect? That must be it. Interesting ....
OK Brian I get the sarcasm,

given the scenario there are a number of possible causes. I am not saying the VR is definitely the problem but it fitted all the listed symptoms.

- A failed VR will cause the tacho to fluctuate
- A dense mixture requires a strong spark to ignite (eg under boost)
- A failing VR can be temperature sensitive

When measuring voltage to determine a VR failure you have the let the battery recharge after starting the car. The Voltage will continue to climb and the high system voltage problem may only manifest itself after the battery has recharged from the start. (This is what appears to be the case in the last post)

Brian I am not going to defend my suggestion any further...( I too have a Type A personality)
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Old 10-28-2007, 02:30 PM
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OK. I just changed the whole alternator, as it what what was available even though it was 400$. A VR wasnt avail and was special order.... thats the way my luck goes... but hey, I dont have the most patience for things either... ha.

Got it in tonight and all of my troubles disappeared... runs perfect, re-adjusted my idle mixture and off we went. No smoke, no hesitation, runs better than it has in 6months.

So, I guess the more dense air made it run better when really cold out... or at least maybe the regulator didnt act up as bad until it got some temp on it... hard to say, but am very thankful for all the insight here and am glad to be up and running!!

thanks again.

Todd

anyone need an alternator core? Probably be fine with the change of the VM....

it wll just sit in my garage until next spring otherwise... pm me if you want it...
Old 10-28-2007, 04:05 PM
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Steve - that wasn't sarcasm.
I was thinking through what you had said while I typed and that was the conclusion I came to. The temperature masked the problem when cold due to the slight change in A/F ratios. I have seen that phenomenon in other aspects of CIS.
The typed word does not translate well at times.
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Old 10-28-2007, 04:41 PM
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Great call on the VR guys,, a problem like that could have been a "stumper" for a while.
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Old 10-28-2007, 05:08 PM
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Quote:
Originally Posted by RarlyL8 View Post
Steve - that wasn't sarcasm.
I was thinking through what you had said while I typed and that was the conclusion I came to. The temperature masked the problem when cold due to the slight change in A/F ratios. I have seen that phenomenon in other aspects of CIS.
The typed word does not translate well at times.
Oops my bad... sorry
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Old 10-28-2007, 06:21 PM
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