Hard starting when hot

[rkvons]

Hi. I have a 1998 Montana/Transport with the 3.4, extended cab, A/C. Had it since new. Know all the work done on it, because I did it myself. Last year did the LIM gasket job, and when I had it apart, changed the plugs, wires, air filter, serp belt, etc. Basically everything under the hood that I could think about at the time. Not loosing any antifreeze any more. But for the last couple of years (maybe 3), it has been getting harder to start when it is warmed up. It will always start when cold, but when hot, it seems to have trouble getting going. You go to start it and it sorta revs up, seems like it realizes it went too fast, so it backs off, almost dies, then revs again. It does this only for about 5 seconds. Sometimes it dies during this "hunting". When it does die, it takes accessive cranking to get it started again. Always during the "hunting", it's like a switch is turned on and it smoothes out perfectly and we're ready to go. It runs like a champ down the road though. I can get rubber shifting into second. It idles smoothe as silk, accelerates great. It really runs good, otherwise. No codes for more than a year. Another thing that might be related. Occasionally the tachometer does not work. I have not figured out any pattern to it not working. It sometimes starts working by turning the car off and starting over. And sometimes it won't work for a few days. All of a sudden, I'll notice that it is working again. I have never changed the fuel filter and will probably do that this weekend. If anyone has any other suggestions, I'd appreciate them. Thanks.

[rkvons]

I forgot to mention. It has about 120,000 miles on it.

[1999montana]

My turn!

I'm told that the crank position sensor can cause this hard starting problem and is quite a common part to fail.

Problem is its behind the harmonic balancer somewhere.

I just noticed this squirrel cage type thing behind the harmonic the other day while wrestling with the plugs (and looking for my utility knife that has now taken up residence somewhere in the engine compartment!).

I'm guessing the squirrel cage has some bearing on how the sensor detects crank rotation and position.

Similar to the old reluctor in a GM of Chrysler HEI system from the 70's and 80's

http://www.autozone.com/az/cds/en_us...rInfoPages.htm

[merc81]

I have this problem on my olds. I have a rebuilt engine and I know exactly where the sensor you are talking about is located under the harmonic balancer. However . . .
Its function is to provide a 24x crank position sensor at speeds below 1200 rpm. If it fails, it sets code 0336 and the system uses only the pulses from the crank sensor on the ignition module.
I don't have any facts to back this up, but I don't think thats the problem. The squirrel cage mentioned is really a ring with bars that rotate in front of the sensor. I could see if something got fouled up in there or abuse from dirt got to the sensor that perhaps it would be a question, but mine looks brand new after I reinstalled everything so I know its not the dirt. Also if the sensor were failing, you would think after a year or so things would just get worse...not my experience.


I believe this could be the idle air control valve sticking. Its easy enough to check by removing the valve and checking it for movement. I have not done anything with mine since the problem is so rare, but that's the first place I'd look. Also, if you have another GM car, try swapping the MAP sensors. This measures vacuum pressure in the manifold and determines idle air settings mostly. The MAF is used for higher rpms. They used to be pretty cheap so you could just go get one for 20 bucks or less, but the cost is over $50 now. GM only makes 3--the difference is in vacuum levels on the engine with one for each of 3 ranges. Unless its a high performance engine, you should be able to just swap them. That way you can determine if the sensor is bad before going and wasting money on one.

[merc81]

Just to be clear, I'm talking about the MAP sensor (mass air pressure), not the MAF (mass air flow).

[rkvons]

Okay, I've ordered the Crank Position Sensor today. I'll have it Monday. But, where is the Idle Air Control Valve. I can take a look at this tomorrow. Also, I do have another GM car. So where is the MAP sensor? Thanks.

[merc81]

The map sensor measures vacuum so its going to be connected by tubing to the intake manifold somewhere--depends on the engine. On the 3.4 in the van, its directly in front of the spark coils in the back, mounted on the intake manifold. Its held down by a metal bracket which, when you try to remove you will find the bolts run back towards the coil pack which gets in the way of the socket wrench.

On my 93 s10 blazer, the map sensor is located right on top of the manifold in the front. They are pretty easy to spot.

The IAC valve is right on top of the throttle assembly--it bleeds in intake air around the closed butterfly so it has to be located right where the throttle plate is. Sorry I don't have the manuals with me to dump you a diagram, but here is photo where I've drawn a red circle around the iac and an arrow pointing to the approximate location where the map sensor is located. This photo is one I had online and shows an engine where the map sensor is not yet installed, and the bracket that is overtop the iac (the throttle linkage bracket) is removed.



Hope this helps. Again, I'm not certain what is causing the problem you have; I don't get it often enough to warrant going after it yet, and as you say, no codes are set by this. Maybe others with this experience can tell us what they found.
I believe you can change out the 24x crank sensor without pulling the harmonic balancer, but its really going to be a tough reach in there. Go in from the cover plate behind the passenger tire--without checking I think that will look right in on that sensor.
John

[rkvons]

Okay, so I've started the repair procedure on this. Still don't know the cause. I took the IAC mechanism off and sprayed it with WD40. It was stiff and charred looking and rinsed up pretty good. It seemed to move more free. I test drove it and it still has the problem. I did not mess with the MAP yet. I did notice something though. Even though the vehicle is completely warmed up, I can turn the engine off and immediately start it without any trouble. It's when it sits for a period of time (30min - 1 hour, for example) that it has trouble starting. Now I did turn the key to on and hear what I thought was the fuel pump charging the system. I just wonder about fuel delivery issues. I wonder if I should check the fuel pressure at the fuel rail when I think it is going to have difficulty starting. What do you think?

[merc81]

I had one that would not start on the first couple tries, only after it cranked for a few seconds and after the 2nd try. Didn't matter if it was hot or not. This was caused by the fuel pressure regulator bleeding down the fuel rail--it didn't leak fuel, but let the pressure escape back to the tank. The engine wouldn't start until the fuel pressure got back up to +50 psi and that took it a few seconds.
This is quite different from what you described; 'hunting' and random idle speeds. Fuel pressure problems would leave you with a lack of pickup (fuel pump low pressure) normally, not idle speed hunting.

You can check the fuel rail pressure at the shrader valve located back by the power steering pump. I don't have the exact pressure on hand here, but it would need to be over 50 psi at all times.

From your first post description, I'd be very surprised if it was a fuel pressure problem. Really thought the IAC would do it, sorry you're not having much joy yet.

[rkvons]

Well I changed the Crank Position Sensor out today, Sunday. It wasn't as bad a job as I thought it would be. I drove the vehicle to the auto parts store and had to wait in line to return the rented pulley puller. When I went out to start the vehicle, it was hard to start. So that seems like it was not the problem. Weird thing though, I disconnected the sensor completely and the car could start. There were no codes set. The tach was not working. I am still waiting to see how it performs and will have to decide on the next thing to investigate.

[merc81]

Only have the MAP left to try. How's your tach work since you changed out the 24x sensor?

[rkvons]

Originally Posted by merc81

Only have the MAP left to try. How's your tach work since you changed out the 24x sensor?

The tach worked after the car was intact. I will go for the MAP sensor next. I am also wondering about the IAC again. Maybe it has trouble when it is hot.

[dewaynep]

I would definitely check the fuel pressure, should be 41-47 psi. Also, while you are checking pressure, check the bleed-down rate. It should hold pressure for at least 30 minutes or so. It sure sounds to me like a leaking injector or a bad temp. sensor. You could put it on a scanner to see what the ecm thinks the engine temp is when hot compared to when cold. A simple test would be to hold the gas pedal to the floor when you try to start it hot that will shut down the injector pulse and clear a flood if it is flooded.

[jester805]

Hey guys-
I recently bought a used 2005 Montana SV6 with 71,000 miles on it. It has the EXACT same starting problem that rkvons is having. I can take it to the hardware store (2 miles away), spend 10 minutes in there, and when I come out it won't start. I have to try starting it 3 or more times before it finally starts and that's with my foot on the gas pedal.

I've had it at the Chevy dealer probably half a dozen times. The service manager saw it happen ONCE, but it hasn't happened for them since then. They keep telling me that they can't fix it until it happens again. They check the pressure every time they start it and it's always within range. I'm getting sooooo angry with them that I can't stand it.

Please keep posting if anyone figures out what the problem is and I'll do the same.

Thanks

[rockwood84]

if the fuel regulator is bad you can check it by pulling the vacumn line off if gas comes out it is bad[the diaphram has cracked]. i has seen the tps slip from true position [there is a notch on the tps] and really mess up the idle before.the map sensor line or hose can crack from age . there is one other crankshaft sensor [7x]which is on the rear side of the engine that works with the one behind the harmonic balancer.but if the plunger is sticking in the iac then it won't work properly.