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1994 Astro Engine Surge at Idle
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mofish
User
Sep 24, 2011, 11:19 AM
Post #1 of 19
(8658 views)
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1994 Astro Engine Surge at Idle
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1994 Astro AWD EXT. 4.3L V6 VIN W, automatic, 120K miles
The vehicle was rough idling and stuttering when shifting. The catalytic converter was making a rattling noise. Smelled gassy.
What has been done so far:
I did a complete tune up; wires, plugs, cap, rotor. I replaced the vacuum hose circuit that comes off of the back of the upper intake manifold. I cleaned the ERG valve.
Chevy dealer: “checked for codes, none found. Checked fuel pressure 62 PSI (good). Ignition timing OK at 2 degrees. Did find low engine vacuum and spark plugs very black. O2 sensor would not come out to perform back pressure test but suspect there is an intermittent blockage in exhaust”.
Midas: New exhaust from just behind the O2 sensor back (cat. conv. and muffler).
Engine diagnostic package: Check fuel tank for fuel and fuel pressure; engine cranking test using analyzer, check compression, check timing belt condition; visual inspection of the wire, vacuum hoses, connections, relays, belts and emission system.
Replaced CMFI central multi fuel injector.
Found low compression on #3 but did not do dry/wet test.
The vehicle ran worse than before and still smelled gassy, but I had to take it home as I was leaving town for a while.
The manager at Midas agreed to do a wet/dry compression test at no charge when I got back to town. I brought it back and the mechanic ended up taking the top manifold back off to recheck the injector they had installed. He found that one of the feed lines had a pin hole in the feed side and was drowning the cylinder when idling and running lean at cruise. They installed a repair kit and retested “all ok at this time”.
The engine runs much smoother and has more power now but it idles very uneven (surges). It also has a dead spot: if I tap the gas pedal quickly the vacuum goes to zero but jumps right back up as the engine accelerates.
I tested the Throttle Position Sensor with a voltmeter and it operated smoothly throughout the range.
I removed the O2 sensor (easily) and tested with a propane torch and voltmeter. It produced 0.93 volts quickly and dropped back down to zero very quickly. The O2 heater circuit tested good.
I went to a junk yard and got both sides of an O2 sensor connector and made a test pigtail that I could wire inline as it is very inaccessible to back probe. The voltage toggles between approximately 0.82 and 0.4 volts.
Vacuum fluctuates between 17 and 21 PSI when the engine is idling (surging) but stabilizes at higher RPMs.
I uploaded a video of the O2 sensor test on Youtube, the link is below.
Any suggestions?
http://www.youtube.com/watch?v=scm5RNK6gHA
I replaced the O2 sensor today and there is no difference.
mofish
(This post was edited by mofish on Sep 24, 2011, 4:47 PM)
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Hammer Time
Ultimate Carjunky
/ Moderator
Sep 24, 2011, 6:42 PM
Post #2 of 19
(8618 views)
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Re: 1994 Astro Engine Surge at Idle
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You may have a bad IAC ( idle air control) but try eliminating any vacuum leaks and cleaning the throttle body first. Remove the intake snorkel, have someone hold the throttle wide open for you and scrub the back side of the throttle plate and surrounding bore with an old tooth brush and some carb cleaner. Be sure to spray some into the small holes next to the throttle plate. That should help stabilize the idle. If it still has a problem, replace the IAC.
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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mofish
User
Sep 25, 2011, 9:58 AM
Post #3 of 19
(8599 views)
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Re: 1994 Astro Engine Surge at Idle
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I will try this. I looked at removing the IAC to clean it but there is some sort of bracket with a vacuum diaphram (?) on it in the way of one of the screws. I guess I just have to bite the bullet and take that off to get at it.
I didn't mention that it runs at an even idle for about 30 to 45 seconds after it is first strarted. Every now and then after it is warmed up it will run even for a brief period. But what is always consistent is the dead spot when I tap the gas pedal quickly: the engine hesitates momentarily--vacuum goes to zero--and then revs up normally.
The mechanic has had the upper intake off twice now to replace and then repair the CMFI so I really hope there is not a vacuum leak there. Is carb cleaner or WD40 better for finding vacuum leaks?
Thanks foir your help
mofish
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Hammer Time
Ultimate Carjunky
/ Moderator
Sep 25, 2011, 10:33 AM
Post #4 of 19
(8595 views)
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Re: 1994 Astro Engine Surge at Idle
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I didn't tell you to clean the IAC. I told you to clean the throttle body.
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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Mr.scotty
Enthusiast
Sep 25, 2011, 6:21 PM
Post #5 of 19
(8582 views)
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Re: 1994 Astro Engine Surge at Idle
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In reply to "Is carb cleaner or WD40 better for finding vacuum leaks?"
Carb cleaner but, I think propane is much better.
If the engine is hot enough the carb cleaner could cause a fire.
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Your only as good as your tools!
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mofish
User
Sep 26, 2011, 10:38 AM
Post #6 of 19
(8576 views)
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Re: 1994 Astro Engine Surge at Idle
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Thanks for the input. I cleaned the throttle body and throttle plate, and I put a new gasket on the egr valve with no improvement. I have ordered an IAC valve/motor online so it will be several days before I can try that. I couldn't find any vacuum leaks with the carb cleaner.
mofish
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Mr.scotty
Enthusiast
Sep 26, 2011, 3:48 PM
Post #7 of 19
(8570 views)
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Re: 1994 Astro Engine Surge at Idle
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Ok keep us posted.
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Your only as good as your tools!
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mofish
User
Oct 2, 2011, 11:35 AM
Post #8 of 19
(8555 views)
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Re: 1994 Astro Engine Surge at Idle
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I replaced the IAC valve yesterday. There was lots of build up on the old one and so I cleaned the pintle seating surface good. Now the van won't come down to an idle at all. It starts right up every time and goes to about 1500-2000 RPMs and stays there. I don't have a tach hooked up so I am estimating the RPMs. I thought it might need time for the ECM to reprogram, but I started and ran it several times to no avail. I cleaned the deposits off of the old one and reinstalled it and got the same result--high RPMs.
I will try to verify that it has operating voltage to it today.
mofish
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Hammer Time
Ultimate Carjunky
/ Moderator
Oct 2, 2011, 12:02 PM
Post #9 of 19
(8551 views)
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Re: 1994 Astro Engine Surge at Idle
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You might want to use a scan tool and check all the temp sensor values when cold and the TPS value. While in there you can see if the IAC is trying to lower the idle or whether it is regulating that idle speed.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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mofish
User
Oct 2, 2011, 12:20 PM
Post #10 of 19
(8548 views)
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Re: 1994 Astro Engine Surge at Idle
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I hooked up a tach and it idles at 2300 RPM (21 LBS of vacuum). I made sure that cruise control is off and the throttle plate is all the way closed. I couldn't find any info on testing the AIC other than a resistance test. I will try it but I doubt that the IAC is bad since it is brand new. I made sure it wasn't over extended before installing and I turned the key on, engine off, for ten seconds before running it the first time per instructions.
Since it idles at such high RPM, I can only assume that the IAC is not closing at all.
mofish
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mofish
User
Oct 2, 2011, 12:24 PM
Post #11 of 19
(8547 views)
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Re: 1994 Astro Engine Surge at Idle
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OK. I don't have a scan tool but I'll see if I can get my hands on one. I did check the TPS with a voltmeter and it operated smoothly through out the range within the voltage parameters cited in my Chilton's manual.
Thanks
mofish
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mofish
User
Oct 2, 2011, 4:15 PM
Post #12 of 19
(8537 views)
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Re: 1994 Astro Engine Surge at Idle
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OK, I unfastened the new IAC and measured the distance from the motor to the end of the pintle with the key off and then on. I saw it was moving, but only about an eighth of an inch. I eroniously thought that if I had it pushed in to a lot less than the maximum of 1 and 1/10 of an inch measurment, that it would come out as far as needed when I started and ran the engine but it didn't. I adjusted it manually to be fully closed when I remounted it and the engine runs at low idle again but with the surging still present. when I put it in gear with my foot on the break it doesn't die and seems to smooth out. I don't think the IAC is the problem.
mofish
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Hammer Time
Ultimate Carjunky
/ Moderator
Oct 2, 2011, 4:19 PM
Post #13 of 19
(8533 views)
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Re: 1994 Astro Engine Surge at Idle
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You're determined to do things your own way which isn't going to tell you anything.
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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mofish
User
Oct 2, 2011, 5:02 PM
Post #14 of 19
(8524 views)
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Re: 1994 Astro Engine Surge at Idle
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I am trying to locate a scan tool that will work on this OBD I engine. All of the ones I have found don't cover the 1994 4.3 L GM. I will keep trying though. I am posting my efforts in the meantime hoping there might be something useful to you there.
I talked to the mechanic who put the injector in and he said someone had adjusted the timing, which you are not supposed to do, and that he had reset set it to zero. He also said the timing was varrying. When I asked what might cause that he said maybe the distributor or the timing chain. He also said the control system was "primative" and that he couldn't even induce it to produce a fault code.
I appologize if it looks like I am not heeding your advice. I am truely a novice with these newer systems and I do appreciate your help.
mofish
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Discretesignals
Ultimate Carjunky
/ Moderator
Oct 6, 2011, 7:13 PM
Post #15 of 19
(8502 views)
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Re: 1994 Astro Engine Surge at Idle
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When the engine is surging at idle unplug the IAC connector. If the surging continues, that eliminates a problem with the idle air control system.
Make sure you spray around for vacuum leaks with the IAC unplugged. You don't want the IAC system compensating while your spraying for leaks. Pay close attention to the gasket between the TBI and the intake. When you replaced the EGR gasket did you make sure the EGR pintle wasn't stuck open by carbon chunks?
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
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mofish
User
Oct 11, 2011, 12:08 PM
Post #16 of 19
(8482 views)
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Re: 1994 Astro Engine Surge at Idle
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When the engine is surging at idle unplug the IAC connector. If the surging continues, that eliminates a problem with the idle air control system.
Make sure you spray around for vacuum leaks with the IAC unplugged. You don't want the IAC system compensating while your spraying for leaks. Pay close attention to the gasket between the TBI and the intake. When you replaced the EGR gasket did you make sure the EGR pintle wasn't stuck open by carbon chunks?
I unplugged the IAC while the engine was surging and the surging continued. I had previously intalled a new IAC valve.
I will continue to try to locate any vacuum leaks. This engine has the CMFI system and I believe the throttle body or air intake is part of the (upper) split intake manifold and does not have a separate gasket.
I did check the EGR for carbon chunks, in fact that is the only problem I have ever had with this engine, other than a fuel pump, previous to this episode.
Other symptoms that I have noticed are abrupt shifting from neutral to drive or reverse and hesitation when automatic shifting occurs while driving.
I have still not located a scanning tool that covers this year of this engine.
Thanks to all for your input.
mofish
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mofish
User
Oct 18, 2011, 5:38 PM
Post #17 of 19
(8456 views)
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Re: 1994 Astro Engine Surge at Idle
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I finally put my timing light on just to check, because one of the mechanics had told me it was varying. When I had the "Set Timing" connector disconnected the timing was at Zero, where it is supposed to be, and stayed steady although the engine still surged. When I reconnected the "Set Timing" connector the timing advanced almost 90 degrees and would vary back and forth by maybe 10 degrees with the engine surge.
I pulled the ECM and could not see anything wrong on the circuit board.
I went to the local Pull-a-Part and found an ECM from a GMC Safari that had all the same equipment (CMFI, Auto, Cruise, AC, AWD). Since the part was only $50 with a thirty day return for store credit I decided to try it, even though all the numbers didn't match.
It worked! The van Idles and shifts smooth again and no more dead spot when I tap the throttle peddle. Halleluja!
mofish
(This post was edited by mofish on Oct 18, 2011, 5:40 PM)
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Mr.scotty
Enthusiast
Oct 18, 2011, 5:52 PM
Post #18 of 19
(8449 views)
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Re: 1994 Astro Engine Surge at Idle
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Cool good job.
--------------------------------------------------------
Your only as good as your tools!
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Discretesignals
Ultimate Carjunky
/ Moderator
Oct 19, 2011, 5:33 PM
Post #19 of 19
(8440 views)
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Re: 1994 Astro Engine Surge at Idle
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Wow, good guess.  Glad to see you had figured out your problem. Who would of thought an ECM would do something like that. Thanks for the follow up.
Closed to prevent hijacking and spammers. Can be reopened upon request from OP.
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
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