Performance Problems after rebuild - 3.6L C4

[sms1305]

After top end overhaul, motor starts & runs well under no load. (New secondary exhaust by-pass sounds good, too!) HOWEVER . . . there are problems now.

Here's the blow by blow.
1. started motor & ran at about 2500 rpm for 20 minutes;
2. with smile on face, drove 5 miles to get something to eat - ran well without any problems;
3. twenty minutes later, car would not start due to dead battery; jumped car & drove to gas station - no problem;
4. afraid that it would fail to start again, I left the engine running while I filled the gas tank (about 1/8 at the time), added 1/2 quart more oil, and left gas station;
5. not more than 100 yards down the road, it started to sputter, cough, and buck; it ran rough the rest of the way home, especially under any kind of load;
6. again, it idles fine & sounds great through the rpm range while standing still;
7. replaced battery with new, test drove a short distance - still rough under load. It also has a mild "hunt" of about +/- 200 rpm after cold start.

Hmmm... As part of rebuild, I - of course - replaced the OEM fuel filter. I had to replace the engine temperature sensor along with it's connection to the harness. All other sensors/connectors appear fine. Obviously, I'm am seriously bummed. I'm thinking I should change out the fuel filter even though it only has 8 miles on it. The fuel tank sat nearly empty for almost 3 months during the work. Is it possible that the lines & tank had a lot of condensation and crud built up or dislodged? O2 sensor??

OK, Porche gurus . . . please help with your ideas. Thanks.

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Steve
'89 Carrera 4
'62 190 SL

[This message has been edited by sms1305 (edited 06-19-2001).]

[paulhagedorn]

Check all the connections to the alternator. Make sure it is working properly. It is possible that the tank has problems or the lines. Cheap way to see if tank has a problem is to replace the fuel filter and cut the old one open to see if anything is in it.

[Kurt B]

it sounds electrical if the bucking is related to the battery incident, otherwise, two independent problems is going to be harder. Try for obvious things like a loose coil connection. I had a bad coil connection problem like that on mine...it came unscrewed from me flopping off the top without connecting it, and it bucked and stalled (doesn't explain failure to crank).

Likewise, check the starter wiring beneath the plastic keeper in the engine compartment, once gain, I thought I had a dead battery, and I had disconnected that (doesn't explain bucking though).

otherwise, if it's just the bucking under load, it sounds like an injector/electrical/fuel outage somewhere. Timing issues are probably out on that. I guess it could be the fuel lines, but wouldn't it gag while revving?...actually, maybe not, usually when you're idling and rev, it sounds decent no matter what's wrong
Sorry can't be of more help.



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Kurt B
1984 Carrera Cabriolet
911 Page
carrera_cabriolet@yahoo.com

[964Cab]

Did you check the compresssion yet ? Also, maybe a valve clearance problem popped up for some reason ?

[BRAINIAC]

The computer doesn't like run down batteries and battery changes. They get amnesia and need to relearn how to walk. Kinda like having a stroke. Mine did similar things after the same situation as you. After a time it started behaving itsself. It also felt much faster after awhile. Be patient for awhile. If it persists for too long, then worry. But for now, drive it.

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Tyson Schmidt
72 911 Cabriolet
92 C-2 Cabriolet

[Bill Wagner]

BRANIAC is right...the 964 series is REALLY picky about voltage levels. That's one problem, but I think you have at least one other problem, and these will likely be electrical. I will address your points in the number that you listed.

1&2. Ok. In it's learning mode, the CPU is happy. Everything is working fine and it's re-learning all it's sensors inputs. Your battery has been disconnected for a while, so the CPU will default to it's learning mode. Not high performance, but generally pretty good.

3. Dead battery means the CPU must relearn everything all over again. Not only that, it's working marginally and is probably confused by the low voltage situation.

4. Why did you put in more oil? Was it low? You just rebuilt this. I assume you filled it up properly. Did you know that if you overfill your car with oil it can make it's way into the intake and then the sparks? If it's oil, it *might* burn off in time. I think the oil overfill is probably a long shot, but it is possible.

5. The CPU could still be re-learning, or what it's reading from your sensors is bad, or you may have oil in the intake from your recent "fill up". These cars are particularly susceptible to a bad electrical output level from the alternator and battery.

6. That's good.

7. You've just reset your DME computer when you replaced the battery (unless you're REALLY, REALLY FAST ). It's going to be flaky for a while. Under normal conditions, your car shouldn't "hunt" much more than +/- 50 RPM, and that should be sporadic.

You just dropped your engine and for all practical purposes, did an overhaul of it.I have several guesses to offer you for what they are worth:

1. Oil has made it's way into the intake, made it's way into the spark plugs. I looked at a car at a dealership where a salesman, unfamiliar with the sump system decided to "fill it up". Oil was literally running all over the place on the car. After that, they had a hard starting, idling, and general rough running. How full did you fill the oil right after you finished the rebuild? If you deliberately left a slight deficit, then this probably isn't the problem, but if you filled it up to it's maximum capacity, you may have experienced this problem. I don't think this is really likely but it is possible.

2. Connections, both electrical and vacuum might need attention. I think this is VERY likely, simply because you had to disconnect and reconnect all of them to do your work. Ask yourself this: Under load, which sensors does the DME monitor and make adjustments based on their input that would cause the car to malfunction ONLY under load? I can certainly think of several, but the O2 and air flow sensor seem to stand out to me, as well as vacuum lines.

3. I think a fuel filter problem is VERY unlikely. Possible, sure, but likely no! My car sat around with little use from the previous owner for over 5 years and no such problems exist. The tank is plastic, so corrosion shouldn' be a problem.

I ain't no guru...take my comments for what they're worth. $50 says it's a trivial problem.

Good Luck,

Bill Wagner

[sms1305]

I found about 1/2 teaspoon of water in a 1/2 cup sample of gas from the fuel filter.

It idles fine, but continues to lack any power when clutched in. When under load, it "pops," sputters, etc., and then stalls.

Any thoughts about cleaning out the system and/or the injectors? HELP!!

Thanks.

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Steve
'89 Carrera 4
'62 190 SL

[This message has been edited by sms1305 (edited 07-19-2001).]

[Early_S_Man]

Steve,

You probably got a load of badly contaminated gas. It would be a good idea to completely drain the fuel system, then refill the gas after you have let it settle out. Your injectors could be cleaned in an ultrasonic cleaner overnight in a solvent such as B-12 Chemtool, preferably heated to 120 degrees F.

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Warren Hall
1973 911S Targa

[Rod Walter]

Steve,

Warren is right. 1/2 of a teaspoon in 1/2 of a cup of gasoline is a lot and no amount is acceptable.

Go to the last place where you bought gas and ask the people there to check their tanks for the presence of water. If there is any water in the tanks they should pay for you to have your gas tank and fuel system cleaned. If no water shows up ask if they had their tanks cleaned recently because of a water problem. Any reputable oil company will reimburse you for water in fuel that you bought from them.

If there is only a tiny bit of water left in your tank you may be able to get away with running a bottle of "gas dryer" through the system. But these contain methanol, which is not recommended by most manufacturers for their fuel systems on a regular basis since it can dry out seals and other rubber components.


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Rod Walter
'88 911 coupe