Hi all...I've been fighting with my 74 2.0 liter recently. It's sluggish and the throttle response is poor. I measured the cold resistance of the CHT and it was 1400-1600 ohms. So...I added a 470 ohm resistor and it's MUCH better. So the question is..should I add more? I understand the cold resistance should be about 2500 ohms so I'm not there yet. Is more resistance a bad thing? Will it confuse the ECU?
Any thoughts?
Thanks in advance...
Mike

Happy 4th of July to all you US guys...

Anyone...anyone...Bueller???

When you add resistance, the ECU thinks the engine is colder than it is and it will lengthen the injector pulses and your engine will be running more richer.

Probably to tune it properly you would need to check the air-fuel ratio in the exhaust.

I need to do the same thing with my 1.8L running D-Jet but my resistance is 150 ohms.

have you checked the ecu and m.p.s. numbers for proper application? 73' 1.7 & 2.0 shared the same ecu but added resistance at the CHT like you did. look this over

http://members.rennlist.com/pbanders/DjetParts.htm

best luck, mike m (the other one!)

Yes, I should do that. My car is a 74 2.0. From what I've read I should have a CHT that measures 2500 ohms cold. Mine reads 1400-1600. So by adding 470 ohms I'm still not right, but the car runs better. Where will I find the number of the ECU?
Cheers
MikeM

As a general rule of thumb... More resistance in a circuit will not harm an ECU. It may make it preform differently. Less resistance to an ECU can cause damage depending on the circuit... Example, High Impedence (resistance = 15-18 ohms) injectors can be used on an ECU set up for low impeedence. Low impeedence injectors (resistance = 2-5 ohms) CAN NOT be used on an ECU set up for High impeedence injectors.

There is a very common mod being sold on Ebay for $30 for most EFI cars. It is simply a $0.15 resistor put in series with the coolent sensor to raise resistance. When done the ECU thinks the engine is cooler and injects more fuel by increasing the pulse width. This is an example of how resistance can make it preform different.

OK...so then the engine will be running too rich? That can't be good.
Keep in mind that the cold resistance SHOULD be 2500 ohms for my car. After adding the 470, mine will only be around 2000 ohms.Considering that, would it be running rich? Or only if the cht measures at more than 2500?
Mike

Is the reading you have with the part on the bench? All connectors disconnected? If indeed you should have 2500 ohms and you read 1500, I would look to replace the part. I would also heat it up on the bench and see if the reading changes.

With your reading AND adding resistance, you will not harm the motor running rich. You may be running spot on if your ECU has been reading low. The worse you will do to your engine is foul the plugs from too much fuel. Running lean is far worse on the engine.

I'm not a 914 FI expert but I have built several aftermarket EFI systems. I think you need further testing AND possibly a new part.

If yours is a 74 2L, then it uses the -012 CHT, which is still readily available at AA, Pelican, PP/Automotion, etc. Just get a fresh, new & correct part & a spare for the future, when they go NLA like the -017 CHT's for my 73 2L!

Part nos. shown in the Porsch 914 PET (free download at the Porsche website under Classics section), & -012 is the Bosch p/n.

Don't waste time screwing with jimmying around parts, since it's a stock D-jet, just get the correct part in there IMHO.

Thanks for the suggestions guys. I'll replace the part when I remove the engine for a re-fresh,hopefully this winter. I've heard the horror stories about breaking them or stripping threads. Maybe I'll shoot" pbanders" an email. Clearly he's the guru. I've printed his stuff and read it several times.
Thanks,
Mike

Mike,
Adding that much resistance to the CHT circuit won't hurt the ECU, however, it will cause a new set of problems. Depending on which CHT you have, the readings will be different. Per Brads site a 0-280-130-003 or 0-280-130-012 should read about 2500 ohms at 68 deg and less than 100 when hot. A 0-280-130-017 should read about1300 at 68 deg. and less that 100 when hot. Adding 470 ohms to the circuit will cause the ECU to never get a "hot" signal. The resistance will never go below 470 + hot CHT reading. So your car will run rich. How much? Only way to be sure is a AF meter. Like Tom T in the previous post, make sure what system your car should have and then check components to be sure they are correct. If they are wrong, get the right ones and you will be better off in the long run.
By the way, the resistor for the 017 CHT is 270 ohms, not 470.
Tom

Tom...thanks for that. I realize the resistor mod is a 270 ohm, but I was experimentimg so I made both because my sensor measured very low. There is a noticeable difference with the 470. I'm thinking that I probably have the cht for a 1.7 car in my 2.0 car. I'm kind of on my own here,no 914 experts in my area. Very few 914's in my area!! I'm hesitant to replace the cht myself I guess. Don't want to break something or strip threads...I've read the horror stories.
Maybe I should just dive in and hope for the best!!!
Mike

Mike,
Sent you a PM.
Yea, wouldn't want to mess mine up either. Probably be OK to remove, but anytime we dive in for the first time we have that "uh oh, hope I don't break it" feeling. Pm suggests alternate temp fix.
Brad's site is a gold mine of info. I use it often when searching for solutions to problems. One thing that can happen is the " when warm, it becomes hard to fire". Not really flooded, but just extra rich. Actually some threads here on that. Seems the CHT cools off faster than the rest of the engine and causes the mixture to be too rich when you first try to start a hot engine. One solution is to floor the gas pedal prior to turning on the key.
Good luck.
Tom

Tom...thanks for the PM. Interesting option. One thing about my car is that it starts right up all the time. Hot or cold. Idles great too. I think maybe I shouldn't mess with it!! I just took a look at the CHT. I can hardly see it. I'd need to remove a bunch of stuff to get to it,including an intake runner. Not sure I'm up for that, but there's no 914 experts around here either.
Cheers
Mike

Happy 4th..

Seems like we got focused on a CHT problem. May be that your TPS is dirty/worn and just needs cleaning and adjusting.
Tom

Tom...that was one of the first things I attacked. The car bucked occasionally so I cleaned and adjusted it and it's fine now. Tested the MPS and it's good too. Tried to set the timing but can't see the marks,so that's on the "to do" list.
Mike