About to run my '87 Carrera off a cliff - still surging and backfiring

[Tippy]

Anyone know what the vacuum reading should be for my Carrera?

I am getting 0.4 (cold) to 0.49 (hot) on my Defi gauges that states kpa X 1000 IIRC.

Not sure what that translates to? 11.8 to 14.5 in hg?

[cjmurph]

I use this online calculator for various conversions.
Online Conversion - Pressure Conversion

Curt

[Tippy]

Thanks.

I have a great converter on my iPhone (struggling to read vacuum/boost gauge), but I still don't know what in hg should be.

[Tippy]

Hello Hello Hello hello hello hello

[kodioneill]

17 to 21 at idle 0 at open

[Tippy]

Originally Posted by kodioneill

17 to 21 at idle 0 at open

Are these Carrera numbers? I could not find in the workshop manual.

So, if I am calculating correctly, I am getting 11.8 - 14.5 in hg.

That is low, but I do have a turbo. Hmmmm.

What does 0 at open mean? If you open a vacuum line or WOT?

[kodioneill]

That's for all combustion engines. 0 means no vacuum with the throttle open. In your case it would be a positive boost number with higher rpms. Regardless of it being a Turbo I believe you have low vacuum. Locating the problem may be difficult but at least you have a starting point.

[Tippy]

Originally Posted by kodioneill

That's for all combustion engines. 0 means no vacuum with the throttle open. In your case it would be a positive boost number with higher rpms. Regardless of it being a Turbo I believe you have low vacuum. Locating the problem may be difficult but at least you have a starting point.

Yeah, I don't the turbo has any effect on the vacuum, although my brother does.

Man, I wished I had actual Carrera numbers since you stated in general. These motors have pretty big valves, big runners, and small bore/stroke making me think they may have lower vacuum numbers than V-8's that I've been accustomed to.

[kodioneill]

I'll take a reading off mine tomorrow and give you the numbers.

[r_towle]

I may have a wierd idea, but it goes along with a smoke machine test for you.

If you remove the tubo inlet pipe and attach a shop vac to the throttle body...well sealed.
Then use a piece of lit incense...or punk...one of the smokey types.

Move that smoking item around in the engine bay...intake gaskets, all vacuum lines etc to see if it gets sucked into the intake....might work to help you find it.

RIch

[Tippy]

Originally Posted by kodioneill

I'll take a reading off mine tomorrow and give you the numbers.

Thanks!!

[Tippy]

Originally Posted by r_towle

I may have a wierd idea, but it goes along with a smoke machine test for you.

If you remove the tubo inlet pipe and attach a shop vac to the throttle body...well sealed.
Then use a piece of lit incense...or punk...one of the smokey types.

Move that smoking item around in the engine bay...intake gaskets, all vacuum lines etc to see if it gets sucked into the intake....might work to help you find it.

RIch

Rich, were you doing any "extracurricular" activities when you thought of this?

[Tippy]

UPDATE:

Checked timing and checked out good throughout the RPM range.

So, I changed the DME 8-position selector switch in the back of the DME to position #2 (2.8% enrichment), the engine ran like crap and did not make much boost or when on boost, it ran terrible.

As it was idling, I was turning the screw back and forth and the motor was running all kinds of different ways.

After "feeling" the detents between each position, I finally found position #1 (1.5% enrichment - 1 click clockwise of position 0).

I took the car around the block and finally, it ran like it did before! Yay!

But wait, not so fast! Took it out tonight, ran great for about 5-10 minutes (zero problems) and then bam, right back to cutting out, surging, bucking, and backfiring when light throttled or taking off from a stop.

I am really close to ripping out all of the FI and putting in MegaSquirt.

This Motronic junk is just too archaic to try and fix IMO. I have done everything that has been mentioned here and by the chip tuner/burner.

[Tronicsworld]

Could be:

CHTS failing when hot

Is it a new style two wire??

[jjeffries]

The Motronic system isn't junk, but requires some understanding and test equipment. Your car ran well and now doesn't. Something has failed or changed. It needs to be looked at, in person and not over the web or phone, by a competent professional. To that end, surely it would be worth paying a real tech at a real shop to look at it. I do not wish to sound snarky in saying this, but blaming the system when you don't fully understand how it works could lead to further frustration and wasted time and effort; investing in a couple of hours at a good shop could be the best money you'll spend. Respectfully, John.

[Tippy]

Originally Posted by Tronicsworld

Could be:

CHTS failing when hot

Is it a new style two wire??

It is the 2-wire design and has checked good at both cold and warm.

The car runs bad at cold or hot.

But, I do say this; there might be a possibility that the CHT sensor is acting up. I remember kind of twisting the leads a little more than I liked when removing and reinstalling after rebuild.

Originally Posted by jjeffries

The Motronic system isn't junk, but requires some understanding and test equipment. Your car ran well and now doesn't. Something has failed or changed. It needs to be looked at, in person and not over the web or phone, by a competent professional. To that end, surely it would be worth paying a real tech at a real shop to look at it. I do not wish to sound snarky in saying this, but blaming the system when you don't fully understand how it works could lead to further frustration and wasted time and effort; investing in a couple of hours at a good shop could be the best money you'll spend. Respectfully, John.

You are probably right, but I've learned a great deal more about this car and one day the entire FI will be ripped out as Motronic is so outdated. It is fine for stock, but I am making boost and over 500hp. Toying with Motronic and boost is a volatile mix as there are no safeguards built in to the DME.

So to that:

1. The idiots at the Porsche dealer here will never touch my car (they cost me a bunch of money on my old 996 by misdiagnosing problems).
2. The owner of a very respectable shop here has told me all the things he looks at.

The only thing I've yet to do is get AFR's or open the DME and look for anything wrong. That is shortly coming.

[Tippy]

UPDATE #5 zillion....

Pulled the DME and took it apart. What I found was a lot of brownish, (looked like corrosion?) rusty looking stuff around a bunch of solder joints.

What is interesting, is it appears it may be bridging two or more joints together.

Environment of the DME in my car-

1. All AC/heat is removed from the car leaving lots of open ducts. Before, when I would wash or drive the car in rain, water would come down onto the floorboard (the ducts have been all closed since rebuild).
2. The DME is not sealed. Not sure if it originally was, but now it is not. It is simply the sheetmeal cover, covering the components and is far from air tight.

So, DME experts, have I found the "golden nugget" regarding the poor environment the DME suffered through for years?

Is this corrosion (I assume) causing the havoc? PLEASE see next post for pics.......

[Tippy]

Forgot pics:

Pic #1 - Whoever installed the chip/turbo before the PO I bought from made this note indicating the 8-position selector was on position #1. Maybe they were adjusting for poor running in the past richening up the mixture?



Pic #2 - Brownish gunk found on a couple dozen solder joints bridging some to 2 or more. I question this the most since it appears to bridge the gap between solder joints and if it will cause havoc.



Pic #3 - Golden color on the board, appearing to be subsurface?

[ischmitz]

The first image shows solder flux probably around the power transistors from a poor rebuild service. The second image shows resin on the digital PCB that was used to coat the PCB back in the days. The flux should have been cleaned but oh well.

However, neither is likely the reason for your issues as far as I can tell.
Ingo

[Tippy]

Argh......so the flux is bridging solder joints, and that is not an issue?

I was thinking it was shorting some of the circuits?

If not a problem, only thing left is the chip I guess.