89 Carerra Hot Start Problem

[JPJIII]

I have an '89 3.2 Carerra, just turned 100k miles, with an intermittent hot start problem. Regardless of ambient temp, when the car's warmed up it occasionally won't start. It cranks just fine but will not run. After waiting an hour or so, it will fire right up as usual. My first thought was DME relay and it was replaced. Problem continued. Next thought a leaking check valve on the fuel pump was loosing fuel pressure causing vapor lock and without doing the fuel pressure checks just replaced the check valve. Problem continues. Finally ran the fuel pressure tests per Bentley and it passed with flying colors, residual pressure after 20 minutes was higher than minimum by a good bit. Now I'm thinking its electrical and not fuel although the symptoms clearly point to fuel issues. When next it failed to start I tried bypassing the O2 sensor with a paper clip thinking that could be sending bogus info to the computer. That was unsuccessful. I began carrying my multimeter with me and when it next failed, checked the voltage on the coil and it had 12+ on both sides. Thinking maybe the computer had those cracks on the pin connections that's been reported on this forum, I disassembled the computer and checked all the solder connections with a magnifying lens. No cracks were found.
At this point I'm a bit stumped. Is it possible for an injector to bleed off fuel to cause a pressure drop and if so how can I test for that condition? Is it more likely some electrical gremlin that responds to heat? Any suggestions short of a 1 pound block of C4 and a 30 second fuse would be appreciated

John

[wcc]

I have the same problem although it hasn't gotten bad enough for me to dive in with both feet like you did. The thing I don't understand about some of the stuff you changed shouldn't make any difference when it's hot or cold. Like your check valve, if that is bad wouldn't you have the problem at a cold start? Same with a possible leaking injector, wouldn't that cause a problem at a cold start?

Did you check your fuel pressure regulator?
There are two relays. The DME Relay and the Fuel Pump Relay. You replaced the DME one but did you check the other one?

Just a couple of ideas. I'll be watching this thread to see what people come up with.

Good Luck!

[JPJIII]

Actually I did swap relays around. As I recall I put the A/C relay in for fuel and fuel in for A/C with no solution to the problem. The A/C runs as miserably as it did before, the car starts when cold, usually when hot and sometimes I get to wait an hour.

The check valve was replaced as the Bently said its purpose was to keep pressure up when the system was shut down and I inferred it was to avoid vapor lock from the pressure drop while hot. When the pressure test/fuel volume test came up ok I figured the pressure damper and regulator were ok which fits the overall situation as the car runs great, except when it won't start. The idea about the leaky injector is maybe some wishful thinking as it would be a lot easier to replace on bad injector than trouble shoot a weird electrical problem.

John

[jbrinkley]

CHT sensor

[wcc]

Here is a link to some more information on the CHT.

Missing CHT Sensor?

[JPJIII]

Jerry,
Thanks for the suggestion about CHT. That was on my hit list of possible problems as well. Is the intermittant issue a typical failure mode for it and is there a way to bypass it when the no start problem occurs? I'd really like to verify the problem and fix it so I know its done rather than throw parts at.
John

[wcc]

Read this post from Chris Bennet:

My vote is for the CHT. Because it is intermittent, a DMM isn't going to help you. The CHT is the white connector on the left side of the motor. There should be a group of 3 connectors near the heater blower motor. The top one is CHT (usually).

The CHT should read approximately 3500 ohms cold, and approx 300 ohms hot. You can "fake out" the computer by substituting a resistor for the sensor.

Go to Radio Shack and get a 330 ohm 1/2 watt resistor (or a selection).

Once the motor is warmed up:
1. remove the CHT connector,
2. bend the ends of the resistor leads over on themselves
3. shove the ends of the resistor leads into the female connector for the CHT and use some masking tape to hold it in place.

Now you should be able to drive around as long as the motor is warm. If you still get bucking, it's not the head temp sensor.

If this doesn't make sense I can take pictures.
-Chris

I got it from this link:
engine cuts out.

[JPJIII]

Bill,
Your description makes perfect sense. I'll visit the Shack and get the resistor. Do a test run to make sure the 'switchology' is good and then plug 'er in when it next craps out on me. If it cranks up at that point then I'll know I've got the problem solved. As this is such an intermittent issue it may be a few weeks before I'll have the opportunity to report back but I'll let everyone know how this works out. A CHT is a lot easier to fool around with than most of the other alternatives I've considered.
John

[greglepore]

Look here as well: Stranded! DME Ignition Guru Help needed

Reference sensor is only used at start. Could have a failure mode where it is failing when hot only.

[JPJIII]

Greg,
I'd considered that from looking at other threads and have been hoping that's not the issue as it appears to be a pain to replace. However from experience on an Austin Healy 3000 MKII a gozillion years ago, its always the hardest thing to fix that breaks and never the easiest.

I've read the threads referenced by you, Bill and Chris. All of that combined seems to point to either/both CHT and reference sensor. As the CHT is the easiest to test I'll do that first. If it works great otherwise the sensor is next in line. Although it's probably an intermittant Reference Sensor issue as there are no running issues only starting. An iffy CHT would as likely croak while running and cause driveablity issues which hasn't happened. An intermittant Ref Sensor only affects starting it appears which fits the pattern.

I'll report back when the problem is resolved.

Thanks fellows.

John

[wcc]

Yes, please report back cause I'm VERY interested in what you find out. My car is does the same thing. I just have to finish up some other things before I start that project.

[DW SD]

Can you borrow someone else's DME computer to plug it in and check that the computer isn't the problem?

Doug

[buttjoint]

CHT really isn't all that hard to replace, Pretty easy !

[rcaradimos]

Head Temperature Sensor Replacement
This thread might help

[dc96819]

Could be a intake leak.

[JPJIII]

I thought I'd bring things up to date. After pondering all the great suggestions to date I've come up with this theory which is a change from yesterday. The Reference Sensor is the most likely candidate for my start probem. It occurred to me that if the CHT was the culpret it would as likely fail while driving as after sitting for a time while parked. As I've not had any problems while driving that seems to eliminate the CHT. With the observation that the Reference Sensor is only used while cranking and otherwise no influence on the engine, that fits the symptoms I've had. With all this in mind I've plunged ahead and will pick up two new Sensors this afternoon and replace them this weekend. If my theory is right I anticipate finding some chafed insulation, maybe cracked tip or some type of wear/tear to indicate that it is the source of the problem. Its also possible that the support might be loose and move in and out of position. In any event as the colletive Pellican wisdom is those things are a maintenance item, there's no evidence from PO documentation that they've been replace this won't hurt anything and has a good probability of fixing the problem. If the digital camera is available I'll provide photos of what I find on the old parts, etc.,etc.

[jbrinkley]

Im curious if you noticed any light smoke out of the pipes when your car was warm. I ask because we got the same deal on a friends car. Cold start fine, no smoke. After car is hot, a little smoking and eventually dies with fuel smoke pouring out the back.
wait a half hour car starts fine, no smoke, once warmed up a little, more and more smoke. We unplugged CHT after starting it up cold and got the same fuel smoke pouring out the back. But it does turn over and start fine except maybe the first five minutes after dying on it's own.

[JPJIII]

Jerry,
Nope. That's not been a problem. I get the, advertised as, normal puff of smoke at cold start on occassion but nothing like you described. From what I've read on Pelican and in the Bentley's your situation sounds like the CHT isn't working and causing the computer to go full rich on the mixture.

I'd be curious about what the plugs looked like after that situation as a too rich mixture would cause them to be sooty black and maybe even wet with fuel. Just how you're supposed to get plugs out of a hot or even warm engine without boogering up the threads I don't know. Maybe the engine trolls would be kind and after it started smoking you just shut it down and cooled off and the plugs would still confirm a too rich condition.

The advice above about using a resistor might be useful and the Bently shows normal resistances for the CHT at different temps which could help dianose a crappy CHT.

One thing about all of is is an absolute certainty. For those lessor individuals that wonder why we agonize over these annoying issues, they don't drive Porsches. If they did they'd know.

John

[fumanchu]

The solder joints crack in the DME and cause intermittent problems. Happens often in 944's because the DME is located under the kickpanel and gets hot.

Worth a look

[jbrinkley]

I didn't think of the plug issue. I'll remember to tell him
thanks