No start after lowering car off of jack stands

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ssgtobvious
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Location: Minnesota

No start after lowering car off of jack stands

Post by ssgtobvious »

Howdy all! Apologies for formatting, I'm on mobile currently.
I have a 1986 635csi that had a leaking oil pan. My tech and I went through the process of replacing that. We had the car on jack stands front and rear. We got the job done and no leaks! (No leaks yet anyway) started the engine and she sputtered to life. Didn't want to idle to well but stayed running with a blip of throttle it would stay running and eventually evened out and stayed idling just fine.

We called it a success and went to bolt the wheels and tires on and drop it on the ground. Dropped the car on the ground and went to start it up and it wouldn't start. Checked the fuel pressure regulator and she was getting fuel. Checked for spark, and no spark on cylinder #6. Main relays looks alright, but I didn't have any known good relays to verify that was the concern. Any more tips and tricks to try and diagnose this?

Much appreciated!
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Brucey
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Re: No start after lowering car off of jack stands

Post by Brucey »

apologies if this is obvious but if you have had the car with the doors open and/or the radio on for a few hours, the battery might be partly discharged. The voltage at the injectors might be further weakened by a slightly bad feed from the main relay. You can have a good spark, good fuel pressure , but when the feed is asked to fire all six injectors at once, it won't necessarily manage it if there is the slightest weakness.

Certain types of battery weakness can give enough amps for cranking but not enough volts to work the engine management.

So if you can, charge the battery fully and then try again. If that checks out, look for anything that might have been damaged during the work. If that doesn't show anything then you are into the usual territory of a 'no-start'.

If you have had multiple failed starting attempts then the spark plugs could be fouled, and the engine won't fire properly. So new plugs wouldn't be a bad idea either.

cheers
~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
ssgtobvious
Posts: 4
Joined: Mon Jul 08, 2019 5:01 am
Location: Minnesota

Re: No start after lowering car off of jack stands

Post by ssgtobvious »

It's funny you mention the battery. Its not a proper battery for the car. It was a used one I got from work and I needed one to get the car to turn over, figured it was good enough for a bit longer. I will try a proper battery and report back! Thank very much for the quick response!
ssgtobvious
Posts: 4
Joined: Mon Jul 08, 2019 5:01 am
Location: Minnesota

Re: No start after lowering car off of jack stands

Post by ssgtobvious »

Brucey wrote: Mon Sep 09, 2019 8:19 am apologies if this is obvious but if you have had the car with the doors open and/or the radio on for a few hours, the battery might be partly discharged. The voltage at the injectors might be further weakened by a slightly bad feed from the main relay. You can have a good spark, good fuel pressure , but when the feed is asked to fire all six injectors at once, it won't necessarily manage it if there is the slightest weakness.

Certain types of battery weakness can give enough amps for cranking but not enough volts to work the engine management.

So if you can, charge the battery fully and then try again. If that checks out, look for anything that might have been damaged during the work. If that doesn't show anything then you are into the usual territory of a 'no-start'.

If you have had multiple failed starting attempts then the spark plugs could be fouled, and the engine won't fire properly. So new plugs wouldn't be a bad idea either.

cheers
So I tried jumping the battery currently in the car. She spins over faster now, but no start. I pulled cylinder number 6 plug to check, and no fuel was on the plug, but it still gets fuel to the rail.
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Brucey
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Re: No start after lowering car off of jack stands

Post by Brucey »

do you have spark in both 'crank' and 'run'? Even if the engine doesn't fire in 'start' you ought to be able to see sparks as the engine comes to a standstill in 'run', especially if you are cranking the engine over without spark plugs in.

There should be fuel on the plugs if you have been cranking, twice over.

first from the cold start valve and second from the main injectors. If you connect a test lamp across the injectors you should see if they are firing or not.

You can similarly diagnose if you have cold start injector function or not.

Note that if you have spark and only cold start injector function, the engine will fire in 'crank' but will soon splutter to a halt. Pre 6/87 Engines need their cold start injector; without it only the main injectors will fire on cranking and a cold engine will struggle to start without the cold start injector.

After a failed start you can tell if the cold start injector has been firing or not quite easily; just remove the induction boot, open the throttle butterfly, and you should be greeted with a really strong fuel smell. No smell at all after some cranking means that there is no fuel and since it is unlikely that both the main injectors and cold-start injector stop working at the same time (they operate independently of one another), then the most likely thigs is that the fuel pump isn't making enough pressure. The fuel pump should run during cranking and should push at least 1 litre of fuel a minute past the FPR.

FWIW a common source of a 'cranks but no-start' problem is that the 'start signal' is missing from the ECU. This signal is generated from a skinny wire that is attached to the starter motor and (IIRC) it appears on pin 4 of the ECU. This signal also appears at the diagnostic socket. This signal looks like 0V until the starter motor turns at which point it looks like +12V. If the ECU doesn't see the signal then the ECU doesn't fire the injectors and/or the spark plugs (I forget which).

cheers
~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
ssgtobvious
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Re: No start after lowering car off of jack stands

Post by ssgtobvious »

Right now I do not have any spark or fuel in the cylinder. From the fuel pressure regulator I have fuel coming through. No spark at the plug. Would a faulty ignition coil cause no fuel injectors to operate? I checked the leads on the starter and they all are in tact. The thing that is boggling me is that I had the car running, put the wheels on, then go to crank it up and nothing. I really appreciate the help so far, I'm hoping it's something simple I'm missing.
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Brucey
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Re: No start after lowering car off of jack stands

Post by Brucey »

the main relay has dual outputs. If you have fuel pressure then at least one of the outputs is 'good' (i.e. supports a load) and the fuel pump relay is closing etc. One possible explanation is that the other main relay output is 'bad', i.e. it may measure 12V but when loaded it doesn't supply current and volts at the same time. So this needs to be tested (under load, not just with a DVM)

Of course there are many other causes for a no-start too. Bad crank sensors, no start signal, bad earth, bad solder joint in the ECU.... there are many possibilities. One would expect that the fault is new and due to something that has been done during the recent work (and clouting the crank sensors is top of the list BTW) but it may just be a coincidence, or you might have just disturbed a slightly bad connection (the start signal wire on the starter motor for example).

So by all means look at things that might have been disturbed but if that doesn't give a result then you need to do a proper 'no start' analysis. Verifying the main relay outputs, crank sensors and the 'start' signal would be top of my list.

cheers
~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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