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1997 Astro runs rough at idle

I’ve taken it to a half dozen mechanics, none can diagnose the problem. From a cold start the engine idles well, but once it warms up and the fast idle setting shuts off, it begins to vibrate quite a bit. I spent $3k on a rebuild that included an all new fuel injection system, new ignition parts (including distributor, replaced at a later date,) new wire to alternator (the old one had cracked insulation,) and a new mass airflow sensor (swapped in from a parts van-no effect.) Basically everything is new except for the sensors (crank sensor, idle air, throttle position, etc.,) and the fuel filter and pump. I’m not getting any codes, and at least two mechanics said it had good fuel pressure. Any ideas? It FEELS like a vacuum problem on a carbureted vehicle, but I had the vacuum harness checked as well, and everything seems to be in order.

I have to assume its been done after all of the work, but hopefully the throttle body and idle air control valve are squeaky clean.

Has anyone checked the temp sensors - air and coolant?

When you say “rebuild” I assume that you mean a full engine rebuild. Has the compression been checked since the rebuild? I’d also have to assume that if it was rebuilt properly then it was out of the van at the time and wasn’t reinstalled on old, who-knows-how-many miles and 17 yr old engine/transmission mounts?

You said it “feels” like a vacuum problem on a carb vehicle - has it been checked for vacuum leaks?

It probably has an EGR valve. Somewhere in trying to sort things out, checking its operation and perhaps disabling it as a temporary diagnostic maneuver might be good.

Plugging it up to a scanner that does real-time sensor info could be helpful.

Yes, the mounts are new, the compression was checked (and a bad valve was located, and the engine rebuilder repaired the valve under warranty, the only good thing I can say about that shop.) They were alleged to have checked out all the vacuum lines last time I had it in (something they should have already done as part of their “diagnostic” efforts, imo.) I have no way to know what they did to the throttle body or air idle or throttle sensors, but they weren’t replaced, and everything looked squeaky clean on the outside after the rebuild was completed.

You’re the second person to mention the EGR valve in an online forum, I should direct my next mechanic to check it out. After all this time and lost income, I’m really tired of this shop; instead of solving my problem (after charging $3k for a rebuild, at least $600 in other repairs that had no effect, and three weeks lost working time for me,) they keep telling me to buy a Toyota (which I’d love to do, except that my Astro is worthless in its currently poor running condition, and I probably wouldn’t be able to get more than $1500 for it…)

What is the likelihood of a bad sensor, and would any (or all) of them contribute to poor performance without transmitting a code? Is the throttle body where should I be looking if it turns out the EGR valve is not the problem? Should I bite the bullet and tear into it myself, and stop paying these clowns to do absolutely nothing? I need to get either another years’ service out of the vehicle, or another $800 in resale value in the next couple of months.

The only reason I know things about cars (and though precious little it is, it’s still more than most people on the street) is that I got tired of being jerked around and over charged.

You can easily have things wrong without having codes set.

A basic repair manual for the car is about $20-30 at auto parts stores. It will probably be Haynes and these pretty much stink, in general - but they are easily accessible, better than nothing, and do give you a lot of the basics. You can also get better ones, though you’d probably have to order online and pay more. If you register at autozone’s website, they actually have free online access to repair info - though it is very basic and often rather incomplete.

I’ve never been under the hood of an Astro van so I don’t know how badly things are stuffed in there. I do know, however, that doing things like pulling and cleaning a throttle body and IAC are generally not difficult. Actually very easy given good access. The same can be said of stuff like checking temp sensors - if one has a basic familiarity with using an electrical meter. Most temp sensors are just resistors and can easily be tested. The issue is usually ease of access.

Anyway, for this issue you want to get familiar with the difference between “open loop” and “closed loop” engine operation - google it. It sounds like yours does fine in “open loop” but not in “closed loop.” The coolant temp sensor is a major input both to fuel delivery and to when it enters closed loop. Then you might want to read up on oxygen sensors - since they sit at the center of closed loop operation. Unfortunately, learning about what they are doing requires a scantool. So you’ll have to see how far you want to go.

One of the reasons I mentioned the EGR is that on most cars it won’t function until the car is in closed loop (warmed up). And it can cause idle problems that ACT like a vaccum leak when it isn’t technically a vacuum leak. And it can easily cause problems without tossing any error codes. Ask me how I know…

I think you may be being a little too hard on the shop. They are probably doing the best they can. Here’s the problem: There’s at least a dozen things that can cause this. Each has to be tested one by one until the problem is found. And you may run out of money before you get through all 12 tests. It’s just the way computerized cars are. When they work, they work great. But when they don’t they can be a bear to diagnose. Especially for drivability problems like this. There’s a few tricks though that might help.

  • Make sure your shop has the manufacturer’s scan tool (not just a generic one, the one specific to this car) and knows how to use it to access and interpret dynamic parameters such as fuel trim.

  • Make sure the throttle body and IAC have had a thorough inspection and cleaning, and the IAC is given a complete functional bench top test per the car’s shop manual.

  • Checking the vacuum system is more than just replacing the hoses that have cracks in them. All the vacuum controlled devices have to be checked for leaks too, one by one.

fyi, I had a problem with high idle speed on my 20 year old Corolla a few months ago, and had to go through all this to figure out a solution. It took some time, but with the help of the experts here, I just kept plugging away and eventually I found the problem. You might find some guidance by reading through that thread, here’s the link, below. It’s frustrating, true, but gasoline engines are 19th century technology, and any problem they have can be discovered and solved. It just takes time. Best of luck.

I hear what you’re saying, but right now the mech is just shrugging his shoulders instead of doing as you’ve both suggested. All he keeps telling me is that all of the “computer readings” are fine, even though he acknowledges that it runs terribly. I would think that his “electrical expert” who spent two decades with a GM dealer would have checked the sensors individually without me having to prompt them. In fact, whenever I mentioned things like ‘I read on the internet that sometimes the throttle sensors go bad’, he would roll his eyes and shake his head and tell me, ‘oh, we won’t know until we look at it, it could be any number of things.’ Well, they looked at it for a total of three weeks, so I don’t think I’m being unrealistic in the expectation that the person who I’ve paid to solve the problem actually do so within that time frame. It looks like I am going to have to take this on myself, not that I have the time or space really, but I’m left with no choice since these hacks have cleaned me out of all my money. Thank you for all the suggestions.

Oh and it is ironic that you mention your Toyota, since the same mech insists that buying a Sienna to replace my Astro will magically make him into a competent diagnostician.

An Astro van is not some kind of exotic, super high tech vehicle. They’re common as dirt, and the systems used on it are fairly straight forward to diagnose.

It sounds like your mechanic simply doesn’t want to perform any hard-core diagnosis

When your mechanic says you should buy a Toyota, what he’s really saying is “Look, man, why don’t you just buy a Toyota? Because I don’t feel like actually diagnosing what’s wrong with your vehicle.”

Perhaps he hopes a Toyota is more reliable, and he won’t have to diagnose anything

That’s assuming you would ever bring another vehicle back to his shop

The fact that it idles fine cold but goes rough once warmed up narrows down the number of possibilities by quite a bit. Obviously, though, it will require something smarter than just looking for error codes.

db4690, THANK YOU! My sentiments exactly. The same powerplant is in MILLIONS of GM vehicles:
This Wiki doesn’t even include El Camino, Monte Carlo, and Caprice, not to mention vehicles from other GM divisions worldwide that may have had them installed.

I hate to appear to be an apologist for your shop mechanic, but he/she has a valid point. Whatever problem your vehicle has, it can be solved. And you could probably solve it yourself if you have the proper information, the necessary tools, an enclosed garage, plenty of time on your hands, and the right tools. But that ignores the cost factor. Instead of working on your own car, you could probably be earning money doing something else, something you are already very good ide, like working overtime on your current job. And With a 1997 Astro, I don’t recall if you said how many miles it has, but probably 150K or more, and with $80-100 per hour labor rates being the norm, purchasing another vehicle may be the most cost effective way to get you back on the road with a well running engine. True, maybe the mechanic is just giving you the run-around, maybe all that needs to be done is a couple more tests, remove the throttle body and IAC for a good cleaning, put everything back together, and that will solve the problem, but don’t discount the possibility the advice being offered has some merit. No car lasts forever. Not and remain cost effective.

But it RUNS. It was just rebuilt, and should, theoretically at least, run WELL. There is something causing the problem, something that can be repaired, it is just a question of finding it. And you’re right, I’m bleeding money not for what they are stealing from me necessarily, but for all the time the vehicle is in the shop, it’s not making any money for me. Take note of my handle: “Taxi Rob.” I know what my vehicle is worth not only in terms of resale value, but also in terms of what I can earn with it. It is my office and my toolkit. It is my livelihood. There is no reason for me to believe that any other used vehicle would yield a better result, should it develop any performance issue that is outside the scope (or desire) of my mechanic to diagnose; there is no “Toyota solution.” God forbid I buy a Sienna and it develops a similar issue, what then? Buy an Odyssey? Where does it end? It should end right now, with a mass-produced American car being repaired by someone who knows how. That is what we pay other people for, to solve our problems, not hem and haw and make excuses. Sorry to rant, but as Taxi Rob I’m supposed to know where I’m going, and as a mechanic, you’re supposed to fix things.

@TaxiRob … No argument with what you about what you say. Mechanics are supposed to fix things. And I can virtually guarantee that if you present your mechanic with a cashier’s check for $40,000 and say “fix my car”, they’ll fix whatever is broken and you’ll be back on the road, and probably they’ll return some change to you too.

Or you could use the $40,000 to buy a new vehicle.

I think that is the mechanic’s point.

@GeorgeSanJose Remove the throttle body and IAC for a good cleaning. I’m sure the rebuilt used parts off of you old engine.

GeorgeSanJose I don’t have $40,000, nor do I have the credit to get a loan for a new car. The mechanic should understand his client’s needs, or get out of the business.

I would hate to venture a guess without knowing what the exact compression numbers are. If an alleged problem with a valve (cylinder head?) surfaces so soon after a rebuild it all becomes a bit suspect.

If a vacuum problem is suspected, intake manifold gaskets, throttle body gasket, and even a worn throttle body are also suspects, not just the vacuum lines.

I think TaxiRob’s issue here, though anyone should straighten me out where I’ve missed things, is that he just had this engine rebuilt but was not returned a working vehicle in the end. If the shop did do the entire R&R, then as far as I’m concerned it is on them to get it all figured out whether simple vacuum leak stuff or worse.

I assume you are not going to do the work yourself. Go to the “Mechanic Files” link above and find a well rated mechanic for you area. A good mechanic saves you money by not charging for unneeded repairs and fixing it the first time. A smoke test will determine if you have a vacuum leak. A compression test will determine if valves and rings are good. If it runs OK, but idles poorly it could be a dirty IAC. A temp sensor would tell it the wrong temp and cause it to run rich and poorly when warmed up in closed loop mode. Lazy O2 sensors could also be a contributing factor. A good mech with a scan tool can monitor the sensors and replace just what is needed.

I’m with @cigroller on this

It sounds like the mechanic has put his foot down and said “Mr customer, I don’t give a . . . . that you paid 3K for a rebuilt engine, plus $600 afterwards. I’m done with you and your van. I don’t feel like diagnosing it. Go buy a Toyota, because I can’t figure out this Astro van. Go away. I don’t want to see this van again.”

Sounds pretty pathetic, actually