Diagnostic Mystery (Crank, No-Start) [SOLVED]

[gsbarry]

Not an expert on this; but, any airflow not through the mass air meter is unaccounted for.

My thought is that this would make your air/fuel ratio incorrect. Meaning lots of air without adequate fuel -> super lean, still no run and unlikely to have any tail pipe fuel odor as the PCM is no opening up on the injectors. Remember, for an easy start, it needs to be rich. The IAC is metered air. The seal leak is unmetered.

The only simple test is carb fluid around the suspected failed seal. While attempting to start with no pedal.

The opened throttle start from earlier would give you a bunch of metered air. Possibly, allowing a closer to the necessary stoichiometric ratio.

I see that line of thinking. Makes sense. That would seem to suggest a gross leak in the gaskets though, wouldn’t it? When combined with the observation that prior to the no-start the engine ran (both under load and at idle) quite well, then suddenly no-start, suggests to me a failure of a more mechanical nature.

Put another way, I would expect an intake manifold leak to get progressively worse over a longer period of time, which would then become noticeable in a poor idle, prior to being significant enough to cause a no-start.

 

[Chris_A.]

I see that line of thinking. Makes sense. That would seem to suggest a gross leak in the gaskets though, wouldn’t it? When combined with the observation that prior to the no-start the engine ran (both under load and at idle) quite well, then suddenly no-start, suggests to me a failure of a more mechanical nature.

Put another way, I would expect an intake manifold leak to get progressively worse over a longer period of time, which would then become noticeable in a poor idle, prior to being significant enough to cause a no-start.

My guess would be to use carb spray to rule out the gaskets. Why, cheep and fairly quick to diagnose. A vaccum leak does not have to be the intake gasket. What about the PCV (both sides) the brake booster, or anything else that could leak beyond the mass air meter including a cracked intake/exhaust manifold. There are things that are likely and things that are unlikely. With the suggestion of intake manifold gaskets, it is that. A possible cause. I am not saying that the IAC is or is not the cause or that the intake gaskets are or are not the cause. We try to figure out what is wrong by eliminating what can be checked quickly and cheaply, first. And then, if that does not turn up the failure, you are forced to do a step by step set of checks to figure it out.

An example of checking for vaccum leaks:
I check for a vaccum leak at the intake gaskets. Need a can of carb cleaner or ether to do. (Simplest & quickest)
No?
I then do a step by step check of everything that uses vaccum to verify that none of them are bad. I need a vaccum gage/pump to run these tests, a list of specifications, and a good idea of what is what and how to run these tests, so that I can run these tests. (In-depth checks)

 

[gsbarry]

My guess would be to use carb spray to rule out the gaskets. Why, cheep and fairly quick to diagnose. A vaccum leak does not have to be the intake gasket. What about the PCV (both sides) the brake booster, or anything else that could leak beyond the mass air meter including a cracked intake/exhaust manifold. There are things that are likely and things that are unlikely. With the suggestion of intake manifold gaskets, it is that. A possible cause. I am not saying that the IAC is or is not the cause or that the intake gaskets are or are not the cause. We try to figure out what is wrong by eliminating what can be checked quickly and cheaply, first. And then, if that does not turn up the failure, you are forced to do a step by step set of checks to figure it out.

An example of checking for vaccum leaks:
I check for a vaccum leak at the intake gaskets. Need a can of carb cleaner or ether to do. (Simplest & quickest)
No?
I then do a step by step check of everything that uses vaccum to verify that none of them are bad. I need a vaccum gage/pump to run these tests, a list of specifications, and a good idea of what is what and how to run these tests, so that I can run these tests. (In-depth checks)

I generally agree with your overall approach here. I'm still not the biggest fan of the carb cleaner method (but it IS quick and cheap admittedly), partly because I don't like the idea of spraying that stuff all around the engine bay, and partly because it may point to a general area but not really pinpoint the leak, which is potentially difficult if it is an intake manifold gasket leak vs a hose in the same vicinity (same problem with the propane method). I do like the suggestion of the DIY smoke machine (first search result is essentially an old soldering iron epoxied into a jar of baby oil with an air pump attached).

I also have a vacuum pump on hand for A/C repair, but I haven't seen anyone using actual vacuum draw to test for a vacuum leak, which I find interesting (it may be out there, just haven't come across it).

UPDATE: The IAC arrived today and is now installed. The IAC was the culprit as suspected. Thanks all for your help and suggestions. I enjoyed the thoughtful discussion here.

 

[Chris_A.]

I generally agree with your overall approach here. I'm still not the biggest fan of the carb cleaner method (but it IS quick and cheap admittedly), partly because I don't like the idea of spraying that stuff all around the engine bay, and partly because it may point to a general area but not really pinpoint the leak, which is potentially difficult if it is an intake manifold gasket leak vs a hose in the same vicinity (same problem with the propane method). I do like the suggestion of the DIY smoke machine (first search result is essentially an old soldering iron epoxied into a jar of baby oil with an air pump attached).

I also have a vacuum pump on hand for A/C repair, but I haven't seen anyone using actual vacuum draw to test for a vacuum leak, which I find interesting (it may be out there, just haven't come across it).

UPDATE: The IAC arrived today and is now installed. The IAC was the culprit as suspected. Thanks all for your help and suggestions. I enjoyed the thoughtful discussion here.

Glad to know that it was definitely the IAC. Misunderstanding on the vaccum, I was trying to explain that there is a leakdown test for vaccum dash pots/motors. This would be in the service manual. These are all in-depth checks. They cannot be isolated via easy-to-use "roadside" checks. An A/C vaccum pump is used, similarly, to test for leaks on the A/C system. Do not use this on any other automotive vacuum system. It is very likely to apply enough vaccum to damage other systems that have a much smaller volume and are intended to operate at much lower differential pressures. You will have trouble trying to damage a well designed A/C system with the pump, so, it is safe to use it there.