Motronic: Causes of poor cold start performance?

[WydRyd]

Hi all,

What can be the possible causes of really poor cold start performance in the Motronic EFI cars?

My car has developed poor performance when cold (e.g: hesitation), but as the engine temps reach the normal operating range, she runs fine.

Any advice/suggestions on what sensors I can check/change?

[DW SD]

Might look at vacuum leaks, too. Sometimes gaps will close as things expand. Since it looks like your car may be modded, (turbo??), there may be other issues, too.

Doug

[WydRyd]

Doug, the car's been turbo'd for years. That isn't the issue.

This only happens under very cold conditions. Started her up this morning and went for a blast and it drove flawlessly.

Last night however was much colder and that's when the symptoms were exascerbated.

I'll check vacuum connections too.

Is there any way to actually test that the Cylinder Head Temp sensor is working properly?

[ausgezeichnet]

I have the same problem. I need to gas it to keep the rev's up for the first 30 sec then is fine. At this point I think is a lack of residual fuel pressure.

[911 heathen]

Where does she idle when cold?

If she stalls easily, check the Idle Stabilizer Valve....

[WydRyd]

The idle actually hunts when cold. Fluctuates between 700-1200 until it warms up, then stabilizes OK. Certainly sounds like a temperature related issue, so I guess the first thing to check would be the Cylinder Head Temp sensor

I think I read something about the CHT sensor reading ~2.2 - 2.8 Kohms? Is that correct?

[brcorp]

Hope an answer is found as I have the same problem.

[WydRyd]

This guide looks interesting.

[stlrj]

Your cylinder head temp sensor is already the updated two wire type, so I doubt that would be your problem.

Seems your problem (bog and hesitation with hunting idle) is caused by a too rich when cold mixture that can only be fixed by getting your air flow meter adjusted or buying a new one.

Cheers,

Joe

[WydRyd]

Joe, I thought the '84-'89 Carrera had the one wire CHT sensor?

My car isn't a 964, it's a 3.2 Carrera (well, it was until I fit 3.3 P&C's in it )

[ChrisBennet]

Originally posted by WydRyd
The idle actually hunts when cold. Fluctuates between 700-1200 until it warms up, then stabilizes OK. Certainly sounds like a temperature related issue, so I guess the first thing to check would be the Cylinder Head Temp sensor

I think I read something about the CHT sensor reading ~2.2 - 2.8 Kohms? Is that correct?

Cyl head temp is very roughly 3K ohms cold, 300 ohms hot.
-Chris

[DW SD]

Some use various forms of "fuel" to diagnose vacuum leaks. I use carb spray or a small propane torch. If the fuel is sucked into the engine at a specific location, engine will change how it runs, signifying a leak. It may be manifold to head rather than vacuum ports or vacuum lines.
I though the CHT sensor either worked or it didn't, but I deifnitely could be wrong.

Doug

[Magnus Rostadmo]

Could be a number of causes. Most of them previously covered in detail on this BB and Rennlist.
Over a couple of years I ruled out most of the causes and had the same thing also after installing a new CHT.
My nose told me she was consistently running rich at startups whenever the ambient temp was below ca + 7. One time when the battery-charger was on giving 15 V it fluctuated like crazy for almost 3 minutes before idling smooth.
With the battery topped up and left for a rest for about 15 min........silky smooth start in minus 20. Hmmmm.....
After some sleepless nights pondering over it I calculated a bit and put a variable resistance in parallell with the CHT. In fact I used a volume switch from a radio with the right ohm range.
Some cold starts to get it tuned and.......would you believe it.... Completely flawless coldstarts when resistance switch was tweaked according to ambient temp.
With the resistance I "fooled" the system to believe it was warmer than it actually were. Well... previously meant my V-regulator were OK because it regulated nice around 14,55-14,6V but I found that as the temp goes down and the battery aren`t fully charged the regulator permitted the voltage to reach 14,9 to 15 the first 20 -30 secs before regulating it down to the max allowed 14,6 and hunting stopped.
I have also tweaked the idle so that it is adjusted 20-30 under the idle rpm range with the IPV disconnected.
Net Result : Flawless under all circumstances.

When voltage rise over the maximum the system is designed to operate at, the voltage signal that reaches the Motronic "brain" is above its maximum the system is designed for. Therefore it acting like its minus 4 C when its + 6. Fuel according to this is therefore injected causing a very rich condition with hunting for the correct idle rpm as a result.
For sure it can be a number of faults, but carry out this easy checks and see if it helps:
- Fully charge battery, let rest for 15 min. Start the engine and see if it is smooth. If so proceed with next step below.
- Voltage at startup when batt isn`t fully charged and under approx +7. Car must not have been started for several hours.
- Check IPV, should be "buzzing" and preferably clean it with some 5-56, carbcleaner or similar.
- Check idle rpm with IPV not in circuit, setting preferably a smidge lower, ie 790-785 (if standard chip) when reading the average rpm.
- Check CO with professional equipment. Adjust if necessary.

I`ll bet there have been many hours spent in the garage throughout the world trying to fix startup-rpm hunt on 3,2`s especially fall - winter - spring seasons when the regulator pumps up the voltage to keep the battery up.
SC`s can also suffer this symptoms but with the K-Jet injection this is another story.

[Magnus Rostadmo]

I guess my post can be somewhat misunderstood as I mixed up my text a little.
What I am saying is that the voltage regulator can be the culprit for mysterious rpm-hunting at coldstarts.

[jbrinkley]

that's great info, thanks

[Lorenfb]

"When voltage rise over the maximum the system is designed to operate at, the voltage signal that reaches the Motronic "brain" is above its maximum the system is designed for. Therefore it acting like its minus 4 C when its + 6. Fuel according to this is therefore injected causing a very rich condition with hunting for the correct idle rpm as a result. " - Magnus Rostadmo -

"I`ll bet there have been many hours spent in the garage throughout the world trying to fix startup-rpm hunt on 3,2`s especially fall - winter - spring seasons when the regulator pumps up the voltage to keep the battery up.
SC`s can also suffer this symptoms but with the K-Jet injection this is another story." - Magnus Rostadmo -

Both are not true, as the both the K-Jetronic Lambda units & Motronic units
have internal voltage regulation and as such are NOT affected by alternator
voltage changes. These units can operate properly over a battery voltage
range between 8.0 volts to 18 volts. As the voltage goes over 18 volts,
both units can become damaged.

Bottom Line: No engineering system designer worth their engineering degree
would design a system to be affected by voltage changes as described in the
referenced post.

"89 Turbo-look Carrera Cab
Protomotive Stage 3 Turbosystem"

If the above is really the engine EFI system, then it's obviously not a stock 3.2 &
"all bets are off". There could be MANY issues, the first being the chip & how
it was programmed.

Check here ( www.systemsc.com/diagnostic.htm ) for diagnostic info.

[WydRyd]

Lorenfb, the chip in my Motronic is based on the OEM cold startup map. Nothing is changed from the factory cold/warm start maps.

My car has been turbocharged for 3-4 years now and this problem has only just surfaced, so it cannot be the map in the chip, as this was static since day one.

As far as I am aware, the only thing that can affect cold/warm start fuel mixture and idle quality (when cold only) is the CHT sensor

[jbrinkley]

maybe not so great.

[Lorenfb]

"Lorenfb, the chip in my Motronic is based on the OEM cold startup map. "

All the fuel maps were changed because of the turbo mod as have other
things. So to compare to a stock '89 3.2 Motronic is not valid.

[Magnus Rostadmo]

First of all, I`m no expert here !
I apologize if I have mislead someone with my false statements!
I only thought I should share some of my "expiriences" that at least cured my cars symtoms.
It is not so easy to come to the bottom of such problems, at least without spending a lot of money when one live in the rurals.
In fact many is just giving up and consider it as a normal bug.
I don`t think this is normal at all and over the years cause the engine to suffer from abnormal wear (at least the P/C) because of unnecessary petrol-wash-out.

My chip is bone stock. The car is a -86 3,2 Euro with no cat.
My CHT-sensor is new and has for instance a reading at approx 2650 at minus 27 C and about 1100 at minus 15 and 850 at approx zero (if my memory serves me well)
As the problem only occured when the voltage-regulator took the voltage up to 15 - 15,1V (as when batt is low because of the cold) and disappeared at the same rate as the voltage slowly fell to 14,6 - 14,55 in about 20-45 secs.
It newer occured when the battery was topped up and the voltage rise to 15 lasted only 3-6 secs and so I assumed that the fault was here.

If I have understood this system right the cold start enrichment function is regulated by the resistance in the CHT-sensor giving a certain resistance thus giving a certain current at a certain voltage as I=U/R.
Thus giving a certain amount of enrichment. Is this correct Loren ! When I changed my regulator the voltage is not going any further than to 14,65 no matter the state of the battery. Because of my "findings" about the hunting only when slightly overcharging (15 - 15,1V) I therefore assumed that the regulator was the culprit.
Loren has enlightened us, and stated that the Motronic itself has a built-in voltage regulator.
I may just have cured the symptom and not the desease ?
Maybe the real culprit is the internal regulator in the Motronic malfunctioning ? If so, will this affect the mixture across the rpm range ?Only when cold ? Hot also ?
Loren ! Chime in and explain how this works !