3.2 Carrera Only Starts With Starting Spray

[steve84]

My 1984 Carrera died in my driveway and would turn over but not start. I haven’t used it much over the past several years and suspected that old gas turned bad. I drained the old gas, replaced the gas filter and added 5 gallons of gas with 5 oz of Chevron Techron complete fuel system cleaner. Same result, turns over but doesn’t start. I took off the air filter, and yesterday, I sprayed in starting fluid and it started right up. I let it idle in my driveway for 20 minutes and then pulled it into garage and turned it off. This tells me that the fuel is now good, the fuel pump is working and the battery is ok.

This morning, more of the same, turns over but doesn’t start. On the first attempt with starting fluid, there was a couple of pops from the engine and that was it. On the second attempt with starting fluid, the engine roughly started and then smoothed out and I let it run for 10-15 minutes. I turned it off and then I was able to immediately restart it. I was also able to restart it at 5 minutes after and 20 minutes after turning off the engine. But after 2 hours, I’m back at the engine turning over but not starting.

At this point, I’m think that there’s a problem with the spark plugs. I suspect that while the spark plugs can generate a spark hot enough to ignite starting fluid, it’s not hot enough to ignite gasoline. So my next step is to change out the spark plugs. I’d appreciate receiving any thoughts or suggestions that you may have. TYIA

[wjdunham]

Sounds like maybe Cylinder Head Temp sensor. You can try disconnect (the white plug in the left rear corner of the engine compartment) and jumpering with a paper clip or 270 Ohm resistor if you have one handy. Not fool proof but may help you diagnose what's wrong.

[darrin]

have you tried swapping out the DME relay? The DME relay contains 2 relays, one of which powers the DME computer, the other the fuel pump. As they age/wear, the DME relay can develop cold soldier joints that can cut of their circuits (and "magically" heal once current/vibrations close the gaps) -- in your case, sounds like the DME relay isn't activating the fuel pump, but that the cold soldier joint is closing after the engine's run for a bit (via starter fluid) -- Another interim step, if you don't have a spare DME relay yet, is to tap the DME relay (it's under the driver's seat) while trying to start the car and see if that causes the fuel pump to activate/engine to start. It's also worth checking whether your '84 still has the old style single wire Cylinder Head Temp sensor and replacing it with the newer (and more reliable) 2 wire version if it does.

[steve84]

Thank you for the suggestions. I just tried swapping out the DME relay and no change. It do hear a buzzing noise when I first turn the key one position which I understand to be the fuel pump engaging.

How difficult would it be to replace the cylinder head temp sensor?

[FrankM_]

Measure the currently installed CHT sensor first. You can disconnect it in the engine bay and measure the ohms.

You can find the correct values in this post.

Verify my CHT sensor test results

For testing purposes on a cold engine you could use a 3.6 Kohm resistor and connect it instead of the real sensor between the pins and see if the engine starts any better ?


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[MIAdragon]

Mine did that, it’s was the cht sensor.

[steve84]

I feel like an idiot asking this, but is there a trick to separating the white connector for the CHT sensor so I can an ohmmeter on it? I’ve been tugging on it and trying to put a small screw driver between it, but no luck. Thanks

[darrin]

if I recall correctly, you first need to remove (from the side) a metal clip holding it together -- Believe I used needle nose pliers, but it's been a while

[steve84]

Lol, thanks Darrin, very tricky; the clip is wire-like and I didn’t see it until I aimed a high-powered light right at it! I’m heading out to dinner now and will give it another go tomorrow morning.

[FA-18C]

If it is cranking, but no start, you may have a bad fuel pressure check valve, any easy but painful replacement. Mine was very hard to start and also had to change out my fuel pressure regulator.

[mysocal911]

Originally Posted by steve84

I feel like an idiot asking this, but is there a trick to separating the white connector for the CHT sensor so I can an ohmmeter on it? I’ve been tugging on it and trying to put a small screw driver between it, but no luck. Thanks

The temp sensor typically fails in the open condition (infinite ohms) resulting in a running engine (warm) producing black smoke, and eventually dying.
The engine ALWAYS starts when cold.

A shorted temp sensor results in a hard starting mode for a cold engine, but it does try to start.

A bad fuel pump, bad DME relay, or stuck injectors results in a total no start, but the engine does start with carb/brake cleaner.

[Paul Shepherd]

How difficult would it be to replace the cylinder head temp sensor?[/QUOTE]

I just did mine along with the speed and reference sensors and it took around 30 minutes to get all 3 out. Car on stands and remove the LR wheel and it’s a piece of cake. Cut the wire to the CHT sensor then you can get a socket over it

[steve84]

Sorry for the delay in following up, various house DIY projects intervened. I tried to use my old multimeter to get a resistance reading, but the values were jumping all around. I ordered a new multimeter and CHT sensor in the meantime. With the new multimeter, I measured 2.5k ohms for the existing CHT sensor and 2.5k ohms for the replacement sensor, oh well. I nevertheless removed heater elbow as the next step in the process of replacing the CHT sensor, and think I uncovered the problem.

The top connector is for the CHT sensor and if you look at the bottom connector, a portion of the end is missing, exposing wires. Are the bottom two connectors for the speed sensors? Thanks in advance for your assistance.

[boyt911sc]

Originally Posted by steve84

My 1984 Carrera died in my driveway and would turn over but not start. I haven’t used it much over the past several years and suspected that old gas turned bad. I drained the old gas, replaced the gas filter and added 5 gallons of gas with 5 oz of Chevron Techron complete fuel system cleaner. Same result, turns over but doesn’t start. I took off the air filter, and yesterday, I sprayed in starting fluid and it started right up. I let it idle in my driveway for 20 minutes and then pulled it into garage and turned it off. This tells me that the fuel is now good, the fuel pump is working and the battery is ok.

This morning, more of the same, turns over but doesn’t start. On the first attempt with starting fluid, there was a couple of pops from the engine and that was it. On the second attempt with starting fluid, the engine roughly started and then smoothed out and I let it run for 10-15 minutes. I turned it off and then I was able to immediately restart it. I was also able to restart it at 5 minutes after and 20 minutes after turning off the engine. But after 2 hours, I’m back at the engine turning over but not starting.

At this point, I’m think that there’s a problem with the spark plugs. I suspect that while the spark plugs can generate a spark hot enough to ignite starting fluid, it’s not hot enough to ignite gasoline. So my next step is to change out the spark plugs. I’d appreciate receiving any thoughts or suggestions that you may have. TYIA

Steve,

You already found the culprit to your starting problem but you were not aware about it. Install a pressure gauge at the test port near cylinder #1 fuel rail and test run the FP. There are several ways to run the FP for residual pressure tests.

Tony

[steve84]

Thanks Tony. I’m reading up on that now in the manual, and will be ordering a pressure gauge shortly.

[steve84]

Long overdue update: I’ve been following the Bentley’s Manual religiously. The first step for measuring fuel pressure is to remove the driver’s seat and check the DME relay. With the ignition key off, I measured 12 v at terminal 30 using the post and nut securing the ECU to the floor as the ground. With the ignition switch on, I measured 12V at terminal 86 using same post and nut as the ground. I confirmed ground at terminal 85 by measuring 12v at terminal 86 again but using terminal 85 as the ground. I also measured 12v at terminal 87. With the engine cranking I measured 10.xx volts at terminal 87b using same post and nut as ground. When checking for the presence of ground at terminal 85b, I measured 8.xx volts at terminal 87b using terminal 85b as the ground. This difference in measured voltage suggested to me that the ground at terminal 85b was flawed.

The ground connections to the engine were corroded and the heat shrink was brittle and flaking off. I cleaned off the connectors and replaced the heat shrink. Unfortunately, that was not the problem and it still doesn’t start. I’m in the process of measuring the FP today and will update shortly.

[steve84]

I think I have finally identified the cause of my crank but no start condition.

I tested the fuel pressure at the fuel pressure port by jumpering sockets 30 and 87b at the DME relay socket and the fuel pressure with the fuel pumping running was fine at 36 psi, and pressure was adequately retained with pump off, indicating that the FPR is fine too.

As noted above the measured resistance of the CHT sensor at 2.5k ohms was within spec. I finally measured the resistance of the speed and reference sensors. The speed sensor, which is the middle one, measured 965 ohms, so within spec. The reference sensor, which is the bottom one, resulted in an OL reading on multiple attempts. As you can see in the picture, on the bottom one, a portion of the end is missing, exposing wires. This is also consistent with my observation that the tach wasn’t moving at all, when I last cranked the engine without it starting.

Thanks for all your help, and I’ll let you know how my replacement of the sensor turns out.