Hot No Idle

[alex9s]

I have a recently rebuilt 74 2.0 engine in a 72 body. When the car is warmed up it barely idles if at all. It seems the hotter it is the worse it idles and sometimes won't idle at all. For example I took it on a 3 hour trip on the interstate and after the three hours I tried to drive around town but it won't idle but just dies. It will start right up after it quits and runs great at cruise. Could this be the head temp. sensor???? When rebuilding the engine I checked the sensor and it tested to be ok. I also increased the throttle up and it helped some but after more miles it went back to no idle. HAS ANYONE HAD THIS PROBLEM BEFORE????

ALEX

[Bleyseng]

could be the idle mix is too rich so it bogs down the hotter the engine. Try adjusting the idle with a CO meter.
Geoff

[3D914]

Alex,

Also check the HTS when the engine is as hot as needed to produce the problem you've described.

Some other considerations:
1) Ditto Geoff
2) Verify good grounds (3 connectors) at back of engine (behind plenum box), and check tranny ground.
3) Try manually grounding the distributor to the case.

Enjoy!

[azkiwi]

If this is D-jet, then try disconnecting the other temp sensor (the one screwed into the intake manifold).

On my old engine when it would idle and die after being very warm, I thought I had an overly rich running system but it would run better with the temp sensor disconnected. I spent the money to replace all of the sensors (I found out the sensor weren't bad, I had set my digital multimeter incorrectly, hence I read the wrong values). The other thing I kept focusing on was that I thought it was too rich to begin with.

When I mentioned disconnecting the temp sensor in the manifold, someone on this BBS helped me see I had been running too lean and not too rich. A simple 160 ohm resistor in line with the head temp sensor and everything ran much better.

Try disconnecting it (the sensor in the manifold that is) as this will make the whole system run a little richer.
...if it runs worse then it may have been too rich to begin with
...if it runs better, then it may have been too lean.

Best of luck.

Alex

[alex9s]

I tried adjusting the idle with a CO meter but the problem with that is I put in an aftermarket cam. It's a "mild performance" cam from webcam. It's made to run with the stock fuel injection. So..... the reading at idle is already off (too rich). I also replaced the CPU when I rebuilt the engine so it might still be a mixture problem. Anyone have any experience with aftermarket cam setups? Should it still read somewhat close to stock? How can you adjust your mixture with a non-stock cam????

p.s. azkiwi what was the reasoning behind the inline resistor for the head temp sensor????

[azkiwi]

Originally posted by alex9s


p.s. azkiwi what was the reasoning behind the inline resistor for the head temp sensor????

the inline resistor allows the engine to run richer than what you can adjust with the knob on the ECU. My idle problem was too lean (after it was warmed up) and could only be fixed by adding the extra resistance. The knob on the ECU adjusts the lean/rich mixture at idle only...whereas the addition of the resistor in line with the head temp sensor will make it richer at idle, part load and full load.

Best of luck

[chrisreale]

What is the web cam grind? Its been known for people to have idle problems with web cams and stock injection.

[pbanders]

Disconnecting the air intake sensor will richen the mixture by about 10% or less, across all running conditions (idle, part-load, and full-load).

Adding ballast resistance to the head temperature sensor has complex effects. For most ECU's, when the CHT resistance drops below about 100 ohms (corresponding to a head temperature of about 210 deg. F), the ECU interprets that the engine is fully warmed up and does not lean out the mixture any more. If small amounts of ballast resistance (less than 100 ohms) are added, the effect is that the warm-up mixture is richened, but the mixture when fully warmed up is unchanged.

If large amounts of ballast resistance are added (over 100 ohms), the overall mixture is richened, but with an unwanted side effect that the ECU continues to change the mixture even after the engine is fully warmed up. Since head temperatures vary considerably under different running and environmental conditions, this varying mixture can lead to some odd drivability problems (e.g. crappy idle right after hard runs, gets better as the motor cools, etc...).

This behavior is shown by the diagram below:



The CTC Vout is the control signal in the ECU that varies the mixture by head temp. Higher values of Vout correspond to richer mixtures. Note that the 037 2.0L setup is quite different, due to the different CTC resistance profile and 270 ohm ballast resistor.

More details on the ECU response to the air temp sensor and head temp sensor are on my ECU web page:

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

Better ways of affecting the overall (idle/part/full-load) mixture are by varying the fuel pressure (no more than +/- 3 psig from the stock setting of 29.4 psig, more=rich, less=lean), and by adjusting the main mixture screw in the manifold pressure sensor. I suggest to anyone who is considering such adjustments that they do them ONLY after having the car characterized on a chassis dyno while hooked up to a shop quality exhaust gas analyzer. All adjustments should be done on the dyno, too. Blindly making these adjustments without reference to power output and actual mixture data is a bad idea .

[alex9s]

pbanders, thanks for the info that really helps. It's much easier to understand in graph form. chrisreale, the web cam grind is #73. Have you heard of idle problems with this one???? You have me wondering about that. I've heard about problems with the high performance web cams but not with this one. After the engine is hot and it quits it's usualy when I let it go from a cruise, put the clutch in and let the idle drop fast. It's like something isn't catching the idle as it decreases. If I gradually let the throttle down it might idle but very very slowly as in 100-200 rpm's. The thing that gets me is that when it won't idle it still runs great, plenty of power and everything is normal. Do you guys think this could be something as simple as a vacuum leak?????? I used to have the same problem on my 72 1.7 but the problem wasn't as bad (on which I used the same head temp sensor as I have on the 74 2.0). Does anyone know a good value to adjust the mixture to with an aftermarket cam installed??

Thanks for all replies!!

Alex

[3D914]

Alex,

Check out these previous links on the subject. I've had the very same problem with my 74 - 2.0 and #73 Webcam. I've concluded that it can work, but every little detail has to be addressed, and there is very little room for variation.

http://forums.pelicanparts.com/showthread.php?s=&threadid=101197

And . . .
http://forums.pelicanparts.com/showthread.php?s=&threadid=98920

If you still have questions after this, shoot me an email, and I'll be glad to fill you in on where things stand. I'm not sure I got back to each of these postings with results.

Regards,

[Dave at Pelican Parts]

It could be something as simple (and as ugly) as the wrong cam choice...

Evidently the Web #73 grind can be made to work with the D-jet injection, but you have to adjust the MPS. Brad Anders' website discusses this, and mentions why it's not a great idea unless you don't have another choice. You may not have another choice.

Bleyseng has had success in adjusting the MPS (and probably the other adjustments in the system) while running the car on a dyno. You may need to take the car in and pay the $$ for an hour or two of dyno time, plus the $$ for time on the wideband O2 sensor that is used to read your mixture. Bleyseng mentions this in one of the other Web #73 vs. D-jet posts.

It is best to use a dyno that can put a given load on the engine, rather than one that only lets you do WOT (wide-open throttle) runs. They tend to cost a little more, but that will let you tune for more conditions than just WOT.

--DD

[Bleyseng]

Dyno tuning is the way to go to fix this in the long run because you are running a non stock cam. Yes the 73 is similar but off just enough to cause problems at you guessed it, idle.

Dyno tuners/operators won't have a clue as to how to adjust/tune your Djet. I have taken over at a local Porsche Club Dyno Day showing the tuners what/how to do it. READ Brads site over and over until you understand the concepts/info. You can do this your self by adjusting the fuel pressure,MPS, timing with a couple of simple tools and the Wavetek meter.
Find a junk MPS and take it apart to see what Brad in saying in the articles, they are really pretty simple.

Once you take the time and alittle money on dyno time you won't believe how great your 914 can and will run.

What ECU did you swap in?
Do all your FI parts match the correct year for your car's FI set up? CHT, MPS,ECU need to match for the idle to work correctly.
Check Brads site for the list of numbers.
Mis matched parts causes a lot of grief with Djet.
Geoff

[alex9s]

Bleyseng, the ECU I put in was a rebuilt unit from partswerks chicago. All the FI parts were for a 74 2.0 and most were new. New injectors, new trigger points, new hoses, rebuilt MPS(auto-atlanta), fairly new fuel pump. The CHT was pulled from my 72 1.7 and tested to be ok. I was told it would work with the 2.0. Is this right??

3D914, thanks for the leads. That was very helpful. I'm having the EXACT problem that you describe except.......the only problem I have with the cold start is right after it starts it will die unless I give it a little throttle. Otherwise the only problem is the hot running no-idle. It just idles like an American car with a hot cam in it but dies.

p.s. I was wondering, if it's running to rich what if I had the throttle body bored out slightly?? Would that throw the ECU off even more?

[chrisreale]

I recommend sorting out your current issues before doing anything else. Make sure the ECU, MPS and CHT all match. Go to Brad Anders web site and compare the parts numbers