H E L P!

[Matt Holcomb]

I've reached the end of my tether!

Okay. This is what we've done to try and fine the cause of the misfiring, which is now occuring lower down the rev range -- I can't rev the engine past 3,500-4,000rpm, under load. The engine revs oscillate at idle now, too, between 800-1,100rpm.

1. Rebuilt the MFI pump
2. Flushed the fuel tank
3. Checked the fuel lines -- fine
4. Checked the fuel pressure -- perfect
5. Gus from Pacific Fuel Injection tested the injectors -- fair
6. New spark plugs (old ones were badly fouled)
7. Auto electrician checked the plug leads -- fine
8. Tried a new coil
9. Tried another CDI unit
10. Checked the CDI plugs for corrosion -- perfect
11. Re-wired the ignition system
12. Tried another distributor
13. Performed a compression test (210 on each cylinder)
14. Checked the valve timing -- perfect

What have we missed here?

My mechanic wants to try a different muffler now as the Monty that's on it had its baffles removed.

But can a muffler cause this?


Matt Holcomb
MFIWDP
PRVC
BOHICA
1974 911 Carrera 2.7 RoW
1975 Mercedes-Benz 280SE
Dedicated Homepage
Engine Rebuild Homepage
Porsche Owners Gallery Profile
Pelican Gallery Profile

[Steve W]

First off, you want to check that you have enough fuel going into the motor under load. If your exhaust has a provision for an oxygen sensor, check the voltage coming out of sensor at the problem regions, idle, and at full load over 3500 rpm. If its below 0.1 volts, you are not getting enough fuel. If its over 0.9 volts, you possibly could be running too rich. If you don't have a O2 sensor, perhaps a mechanic can put your car on a rolling dyno and do a exhaust analysis. After ruling out fuel as a problem, then you can be sure that your ignition is the problem. I don't think the Monty is the problem though.

[Jim Richards]

I've read a number of posts indicating that exhaust systems can have an impact on the performance of the MFI system. Back pressure is this issue if I recall correctly. If it's discounted by temporarily going to a stock exhaust system, you can focus on the standard MFI troubleshooting procedures.

[thrown_hammer]

I have a high RPM miss and my Mech. suggested the muffler too. My muffler is designed for an MFI car and I have CIS. If we lived closer we could swap mufflers and be done with it.
Let us know if that was it.

[Early_S_Man]

Matt,

I've heard the 'legend' about MFI and mufflers, too ... and have a few doubts about its' validity. My car came with a Manta/Ansa style muffler, and ran fine (except in HOT weather with a bad Beru connector) ... which I replaced with an OEM muffler the next year, due to rusting problems! You don't get much different than the OEM and those Italian designs, yet my MFI gave no indication of problems! It did seem to have better bottom-end torque with the OEM banana muffler, though!

Sure would be nice to have exhaust gas temp and cylinder head temp readings, along with % CO readings on a dyno when the problem occurs ... along with an ignition scope to monitor the Voltage waveform pattern at the plugs ...

[Sean Hamilton]

Originally posted by Early_S_Man
.........ignition scope to monitor the Voltage waveform pattern at the plugs ...

Did the sparky do this?

[Steve W]

'Engine misses under load' - sounds possibly like you may have a ignition wire leak or a bad ignition module. When the engine is under load, compression is at its highest, the most difficult time for a spark to ignite. So if there's a wire leak, it will arc out to ground, or if the ignition module is weaking or tiring, there is not enough juice to 'bridge the gap'.

[pwd72s]

My problem with a less restrictive muffler was the development of a flat spot...not a miss, but a flat spot, the car simply didn't want to gain speed as well as it did with the stock muffler.

[911pcars]

Matt,
The suggestions by Warren should have been performed by your tech early on. No disrespect to your wrench; while wholesale parts replacement may work in uncovering a malfunction but at what price? Hmmm, a spark misfire caused by a gutted muffler? A plugged muffler might limit the rpm, but it wouldn't cause the engine to misfire like you describe. The plug wires were checked okay (I assume with an ohmmeter). Have you observed the engine compartment at night? Look for high voltage leaks to ground around plug wires and coil tower.

Just MHO,
Sherwood Lee
http://members.rennlist.org/911pcars

[Randy W]

If the injectors are only fair that could be a problem. Check the spray pattern against a new one to see what you are missing. I had two injectors that were not able to be cleaned up sufficiently and when those were repalced my MFI S ran much better.

As for mufflers, in my experience the absolute best muffler for an MFI engine is an early 911 stock muffler. I have direct experience with it outperforming the following in driveability and power: Bursch, Monty, Triad.

Montys are well known for not working well with MFI engines.

[Jim T]

I don't know how much the muffler makes a difference, BUT,

my buddy with the mint '73S put on a Monty around 5 years ago. He never could get it to run right. Wasn't a misfiring problem, though, was just getting it to run right throughout the power range was impossible.

With the stock muffler back on, it was fine.

The Monty was a $700 garage wall hanging for many years.

He did all sorts of things with the Monty and trying to get it to run right, including corresponding with the Big Kahuna (Bruce Anderson). Some of the letters were printed in excellence.

They believed it was not really a backpressure issue, but an exhaust tuning issue. My friend even lightly tack welded some baffles to the end of the Monty (removed later) so he could adjust the "backpressure" but it made no difference.

[Superman]

Like Warren, I doubt that muffler issues would be this serious. I don't think the muffler is the problem. It sounds like a fuel delivery problem. A fancy scope would be nice, but for us backyard grease monkeys, the first step is to verify ignition. Sounds like you've largely done that. Once you are certain that the ignition system is working properly, and the problem persists, then you know it is a fuel delivery problem.

[Early_S_Man]

Until proven otherwise, I tend to think it is an ignition problem.

Have you checked the distributor to see if the centrifugal advance mechanism might be sticking? I think a check of the advance curve on a distributor machine is called for! You might consider getting a replacement Bosch 0.231.184.004 distributor ... I don't think they are very expensive. It is a later version than the one that came on your engine originally, I believe, but has the same specs.

[Matt Holcomb]

Guys,

Thanks for your ideas. I'm going to print out this thread and take it with me as soon as we're ready to resume the investigation.


Matt Holcomb
MFIWDP
PRVC
BOHICA
1974 911 Carrera 2.7 RoW
1975 Mercedes-Benz 280SE
Dedicated Homepage
Engine Rebuild Homepage
Porsche Owners Gallery Profile
Pelican Gallery Profile

[jeff m]

I didn't see pump timing on your list, (you reinstalled your pump)
But I would vote for ignition.
Jeff

[WydRyd]

Matt, I have to say I concur with the other fella's here re: possible ignition issue.

My Carrera use to miss severely after a wash (water on dizzy/plug leads). Once it dried away, the miss went away. Now that I have a massive IC above the engine, it shelters the dizzy from water

I'm not saying you have a wet dizzy (LOL), but the symptoms you describe certainly sounds like an ignition issue.

OR, it could be a vacuum leak somewhere on the intake side?

I guess John would have covered every possibility there

I strongly suggest you get it on a dyno and hook up a number of analysers to try to ascertain what the culprit is

[Paul W]

Hi Matt

About now I'd be thinking about seeing an ignition specialist for some diagnostic work - John doesn't have all the goodies for testing this sub system. Failing that sell me the engine... I'll get it sorted and be able to keep up with Merv!

Good luck

[Matt Holcomb]

Merv,

I'm with you -- it has to be the (complex) ignition system.

Paul,

John uses a guy who has all the equipment to find out why -- and where -- the ignition system is breaking down. We're lucky that this guy is close to Spyder as the car is barely drivable. I don't know how I made it back from South Melbourne yesterday! John said that I must have been driving "on the idle"!


Matt Holcomb
MFIWDP
PRVC
BOHICA
1974 911 Carrera 2.7 RoW
1975 Mercedes-Benz 280SE
Dedicated Homepage
Engine Rebuild Homepage
Porsche Owners Gallery Profile
Pelican Gallery Profile

[CamB]

Let us know how it turns out (of course!).

I can make my BMW not rev past 5000. Basically the dwell changes with RPM to the point where it is so close (or far??? - can't remember) that I get points bounce. It won't get past 5000rpm if I set the gap per the specification (60 degrees) at 1000rpm but if I set it to 50 deg it runs up to 6500rpm no problem.

What it felt like was that the car just wouldn't keep accelerating.

Easy to check by revving the engine with the dwell meter attached. Still took a while to figure that one out

Cam

[911carrera]

check the catalyc convertor,if you have one ,for restrictions:eek

Robert: