Engine Dies after long downhill run

[adamj]

Hey guys, wondering if anyone has experienced this before and if anybody has an idea of what could be causing this...

After coasting in a lower gear going down hill (i.e. engine braking) the engine dies after putting it into neutral.
This has happened a few times and all times at the same circumstance:
- at high altitude going over a mountain pass
- specifically on the descent, where little or no throttle is used for a prolonged time.
- Then, when I need to make a turn or slow down, and after I go into neutral, the engine just shuts down.
This is a bit scary sometimes because there's no power brakes or steering (especially down a steep hill). But, I am able to start it up after I notice the engine has died, and it starts right back up no problem at all.

Any ideas?

(Fuel delivery is TBI if that helps narrow it down)

 

[1987 GMC Jimmy]

If you're changing altitude, a good bit at that, then I would start with your MAP sensor as a place of malfunction because that's what communicates air density to the computer in order to devise accurate fuel metering. Have you had any trouble codes, or do you have any other run issues?

 

[bucket]

Could also be a lazy/sticking Idle Air Control valve.

 

[adamj]

Check engine light came on a couple times when going above 11,500 but I didn't have a chance to check the code with counting the light flashes. Light went off after going lower.

I will give that IAC and the MAP sensor a look. I have not gotten into the sensors on this thing yet...

Also thinking about rebuilding the throttle body. Is that something that needs to be done, kind of like a carburetor?

 

[1987 GMC Jimmy]

It's really a thing only if it's leaking somewhere or if your fuel pressure is wrong. Even then, it's super easy. The only other thing is if the fuel spray is not ideally conical or if the injector is sticking, you'll either have to send it off or get new ones. But that's only if you're running nastily rich. I would check those codes first. They're still in your computer if you haven't unplugged the battery since you've had issues.

 

[adamj]

Thanks Jesse, I did not know codes were stored. How long do they stay there? Check engine light is no longer on, but it still has a memory to store codes?

I went out, checked the codes and got these 3:

12 = (just means I'm in diagnostic mode)
24 = vehicle speed sensor (vss) or circuit (might be related to stalling out when rock crawling?)
34 = MAP sensor signal voltage low

So you were right on about the MAP sensor!

 

[1987 GMC Jimmy]

Yes, as relatively primitive as it is, it will remember them. You can store up to three, and they can stay there as long as your battery stays charged and more codes don't succeed the ones already in there. I would first check the vacuum lines associated with the MAP sensor to see if they're slashed, snapped, or disintegrating. I have two vehicles of a similar age and combined, I've probably had a dozen vacuum leaks. Some small, some big. I would make sure the sensor is plugged in tight. If neither of these actions fruit anything, I would make sure your net manifold vacuum is nice and high because that can affect the MAP sensor. Like at least eighteen inches of mercury. Do you have a vacuum gauge? Very comprehensive tool. If you still find nothing, I would assume the MAP sensor is bad and order one online because you'll pay less (unless you need it ASAP). One more thing. You said you were still somewhat aloof on the whole array of TBI sensors. It's a little rectangular box with two screws bolted behind the EST module. By the way, I'm not sure about the VSS malfunction. Have you had the cluster out? It may just be unplugged, but it works in conjunction with cruise control, IAC, and ignition timing in your Burb if I'm not mistaken.

 

[adamj]

Thanks for the info!

I will get my vacuum gauge out and get a reading. It probably won't be up by 18" Hg since I'm up at around 6k elevation, and the engine is not that new, but would be good to see how everything else is running (ignition, valves, etc.).

I will check out electrical connections and vacuum lines to MAP sensor, but the engine runs fine, no stumbling or anything like that, just the scenario I described above at altitude. I have another MAP sensor that I know works, I can swap that in once I confirm vacuum lines are sound, and see how it runs then.

For the VSS, I believe it is related to when I had my cluster out and swapped to full gauges from idiot lights. I lost the small screw that secures (what I believe to be the VSS sensor) to the back of the cluster where it mounts up near the speedo. I used another screw but don't think it is lined up properly. Been driving on it like that for 10k+ miles, and haven't noticed anything strange (besides downhill stall). I don't have cruise control, is it still important?

One more thing, what is the EST module? Has to do with electronic ignition?

Thanks!

 

[1987 GMC Jimmy]

Well, it sounds like you've got everything under control. Was thinking sea level vacuum numbers. Plumb overlooked you're at high elevation. The cruise could be other things like vacuum lines, bad electrical connections, servo disconnected from the throttle body... Just stuff like that. If you really wanted to see what your VSS was doing, you could easily make one of those serial data cables so you could interface with your truck's computer with a laptop for cheap. I did, and it's a very neat tool. Anyways, I digress. And yeah, that's what the module is for. I can't give you a technical definition of what it does because I don't know, but it does work between the computer, distributor, knock sensor, VSS, and whatever else to know when to advance or retard timing. I gotta say you have a really nice Suburban. That interior job was primo stuff, and you've been pretty thorough with everything else.

 

[mr heavy-chevy]

i had the same problem on a ford ranger
it was the idle air control valve
a bunch of carb cleaner fixed it. B12 is my favorite brand.

 

[adamj]

Haven't gotten around to diagnosing yet, but that could be it too. It would make sense: the valve would be completed closed for a prolonged period of time and then when the rpms drop down it fails to open back up and the engine dies?

 

[Georgeb]

Haven't gotten around to diagnosing yet, but that could be it too. It would make sense: the valve would be completed closed for a prolonged period of time and then when the rpms drop down it fails to open back up and the engine dies?

That makes sense to me. It would be nice to be able to log data while it happens to see what the IAC and MAP are doing.