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| ChuckK | 91 L98 "huffing " at idle. | ||
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Guru Newb
 
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I have a 91 L98 with 90K miles.
Last week it started to "huff" (gain 200-300 rpm at idle every 3 to 5 seconds, as if the throttle was pushed. ) I checked for codes but found none. Nothing had been done to the engine prior to this condition starting. I have checked the obvious things, vacuum leaks, plug wires, cap and rotor, sensor connections -- no luck. Under load (acceleration) runs fine. Thanks in advance for any help. |
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Posted on: 2010/7/20 2:14
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| Matatk | Re: 91 L98 "huffing " at idle. | ||
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If you have no vacuum leaks, a common problem that will cause problems is the IAC (idle air control) valve. This is located under the throttle body (square 4 wire sensor). Over the years, it becomes caked with carbon deposits. You can remove it and the carefully clean it. You should also remove the throttle body and clean it well, including the passages underneath (remove the plate). After that you can reinstall and reset the IAC and see if that corrects your issue.
Matthew |
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Posted on: 2010/7/20 2:19
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2002 EBM convertible, Magnusson supercharger, cam, headers, etc. 1989 Corvette...RIP |
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| Vetron87 | Re: 91 L98 "huffing " at idle. | ||
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Elite Guru
 
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My 87 had a surge problem about a year ago. Carefully checked for vac leaks, did a complete tuneup dist.cap, wires,ignition mod, rotor.
Matthew and Andy both suggested IAC. Cleaned it reinstalled runs great. You would be surprised how fast they carbon up. Once a year maintainence routine now. |
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Posted on: 2010/7/20 2:29
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| Tre57 | Re: 91 L98 "huffing " at idle. | ||
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What is a safe method for cleaning the IAC?
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Posted on: 2010/7/20 3:09
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| BillH | Re: 91 L98 "huffing " at idle. | ||
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The Stig Moderator
 
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Pull it (obviously), the FSM says carb cleaner is OK bit no solvent with methyl ethyl ketone. But I prefer throttle body cleaner. Use a small brush to remove any carbon. The FSM also states that you shouldn't push or pull on the pintle. You can put a little lube on the gear, I use distributor points cam lube because it can handle the heat. FSM says to put a little oil on the O-ring to reinstall. Also check the port in the TB for carbon buildup. To reset the pintle position (usually for installing a new IAC). Depress the accelerator slightly, start the engine for 5 seconds, turn it off for 10 seconds. Then restart and check idle speed. |
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Posted on: 2010/7/20 13:10
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Every man dies but not every man lives. |
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| Tre57 | Re: 91 L98 "huffing " at idle. | ||
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Thanks Bill, I knew I had read somewhere that some chemicals would damage the IAC. Thanks for the clarification and the additional instructions. I will tackle this one soon.
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Posted on: 2010/7/20 20:04
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| BillH | Re: 91 L98 "huffing " at idle. | ||
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Just a note, this works sometimes but not always. Some guys report that cleaning didn't help and they had to but a new one but who knows how they went about it. Some guys have had good luck cleaning. |
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Posted on: 2010/7/20 21:48
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Every man dies but not every man lives. |
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| ChuckK | Re: 91 L98 "huffing " at idle. | ||
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Well, I did the IAC thing. Removed the Throttle Body and performed a complete cleaning of the IAC valve and TB. Reinstalled ---no help. After warm-up, engine idle go into an unstable mode. Since this condition only occurs after warm-up, does that rule out ignition items? What is the best way to test for vac leaks? Dealer wants $2,800 for new injectors, plugs, points, etc. (can it really be the injectors??) I'm running out of ideas.
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Posted on: 2010/7/21 19:46
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| Woodstock | Re: 91 L98 "huffing " at idle. | ||
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I had a case where a defective knock sensor would cause this symptom. The only thing we could see was a delayed knock retard at full throttle, when we made a recording with diagnostic software. No one should be without it, TunerPro that is
 Only symptom was a pulsating idle after the computer turned into closed loop. No symptoms at full throttle or during regular driving. Another case was a too small batterie in a TransAm with TPI. Battery was only 34AH and couldn't balance the need for current at a stop light. The alternator was OK, just not enough juice from the battery. |
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Posted on: 2010/7/21 21:19
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| Tre57 | Re: 91 L98 "huffing " at idle. | ||
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Well that sucks. In the middle of doing the samething on an LT-1. |
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Posted on: 2010/7/21 22:20
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93 AT Coupe |
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| Matatk | Re: 91 L98 "huffing " at idle. | ||
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Well, you changed the equation. This is new info. I would check fuel pressure and ohm your injectors. How old is your 02 sensor? If it it going into closed loop when warm, that's when the 02 sensor starts giving feedback to the computer and can make changes. Look into those items. Matthew |
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Posted on: 2010/7/21 22:29
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2002 EBM convertible, Magnusson supercharger, cam, headers, etc. 1989 Corvette...RIP |
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| RickAnthony | Re: 91 L98 "huffing " at idle. | ||
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ASE Master Certified
 
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I would check your fuel pressure regulator, fuel pressure is good to know, but what I would specificly check is to see if the the diaphram has a leak, it may be leaking fuel into the vaccum line attached to it, sometime you can pull off the vac line and see it's wet with fuel, also sometimes you may only see the leak if you put a temporary small piece of clear tubing between the regulator and the vac. line..
the huffing sounds like a surging issue possibly? typically this can be caused by a fuel control issue, ie vac. leak, dripping injectors, fuel pressure issue, O2 sensors, other fuel control sensors ect.. One of the things that leads me to a fuel press. reg. is that it seems to run bad after warm up since you need more fuel when the engine is cold and less when warm, along with a small amount of extra fuel may not be noticed drivability wise when your accelerating, when the eng. is wanting more fuel.. just my .02 -=Rick |
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Posted on: 2010/7/21 23:41
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My Automotive Blog autoserviceupdated.blogspot.com CORVETTE SERVICE & REPAIR RickAnthonyMotorsports.NET |
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| ChuckK | Re: 91 L98 "huffing " at idle. | ||
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Guru Newb
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91 Convertible,
Small world - my 91 is also a ragtop, thanks for the tip on the knock sensor, hope that's it. CK |
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Posted on: 2010/7/22 2:58
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| ChuckK | Re: 91 L98 "huffing " at idle. | ||
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Rick, Thanks for the ideas, I will start checking tomorrow. Just noticed tonight that the oil pressure gauge jumps around between 20 - 60 psi when the idle goes unstable. CK
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Posted on: 2010/7/22 3:04
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| ChuckK | Re: 91 L98 "huffing " at idle. | ||
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Matt, Thanks. The shop said that the injectors ohmed between 6 to 9 and all injectors were within spec on the balance test. My guess is the O2's are origional. Thanks for the tips. CK
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Posted on: 2010/7/22 3:25
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| Matatk | Re: 91 L98 "huffing " at idle. | ||
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Well, I think this is a test you need to do yourself. If the shop said 6-9, that is not correct. Do you have a digital voltometer? If not you can buy one for around $10. This test takes 5 minutes. Take the clips off the fuel injectors and use the probes on the injectors themselves. You should see a resistance right around 16. A little bit different is nothing for concern. But if you see one that is way off, that's a problem. Do this test with a hot and cold engine. Also, O2 sensors are a maintenance issue that gets neglected. Non-heated 02s should be replaced every 30,000 miles. Matthew |
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Posted on: 2010/7/22 11:38
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2002 EBM convertible, Magnusson supercharger, cam, headers, etc. 1989 Corvette...RIP |
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| ChuckK | Re: 91 L98 "huffing " at idle. | ||
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Matt, Tkx for your comments. I unplugged the O2s to see if it would make a diff - no help. I have a DVM so will ohm the injectors tomorrow and run the DVM test on the O2s also. If no luck, I will move on to the fuel pressure test if I can get my hands on a gauge. CK
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Posted on: 2010/7/23 0:50
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| Matatk | Re: 91 L98 "huffing " at idle. | ||
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It is very difficult to test an o2 sensor with just an ohmeter. You have to be running the car (think 500* exhaust) laying on your back underneath it. Not practical. You are better off to use a datalogging software and a laptop for that. You can also remove the sensor and stick it in a vice and heat it up, but again that's a PIA. And as I said before, this is a $30 maintenance part and should be replaced if it is original.
Matthew |
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Posted on: 2010/7/23 11:46
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2002 EBM convertible, Magnusson supercharger, cam, headers, etc. 1989 Corvette...RIP |
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| RickAnthony | Re: 91 L98 "huffing " at idle. | ||
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ASE Master Certified
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actually you don't have to even get under the car, you could scope it or use a DVOM (though it's not the prefered choice)... just use a "T" pin and back probe the PCM although I forgot the year of this one, if it's early it's a pain as it's under the dash if it's a later C4 under the hood it's a piece of cake. With a scope you can actually see what the sensor is doing like a graph you can see if it switched quickly or if it's lazy and slow, the DVOM won't show you that.. the scan tool through the pcm only shows you what the PCM is thinking it's seeing.. and may not be quick enough, also unless it graphs you will only be looking at numbers.. again though if you suspect the O2 is bad just replace it... but I would check the fuel press reg first..
-=Rick |
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Posted on: 2010/7/23 12:02
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My Automotive Blog autoserviceupdated.blogspot.com CORVETTE SERVICE & REPAIR RickAnthonyMotorsports.NET |
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| ChuckK | Re: 91 L98 "huffing " at idle. | ||
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Guru Newb
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Matt,Rick Well here is the data on the injector ohm's and O2 sensor test in mv :
Cyl# Cold Hot 1 13.4 8.5ohms 2 16.8 17.3 3 10.6 8.6 4 9.3 9.3 5 10.5 8.8 6 8.5 6.7 7 17.1 14.5 8 8.8 9.1 O2 sensor test in mv: 390mv at cold start 570mv at idle drop 710mv idle started to "pulse" 780mv Hot (after 10 mins idle, held this value for 3 mins) Note: mv rate if increase appeared to be constant from 390 thru 710 and slower to 780. So what does all this tell us?? I hope to perform the fuel pressure test over the weekend. CK |
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Posted on: 2010/7/24 3:44
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| Matatk | Re: 91 L98 "huffing " at idle. | ||
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Your injectors are bad and should be replaced as soon as possible. Very common with the multechs from our model years. Check with Jon at FIC for good replacements. I have a write up in the tech library.
Matthew |
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Posted on: 2010/7/24 18:58
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2002 EBM convertible, Magnusson supercharger, cam, headers, etc. 1989 Corvette...RIP |
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| bogus | Re: 91 L98 "huffing " at idle. | ||
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Grand Imperial Pooh-Bah
 
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DING! Bad injectors for sure.
Those things are toast. Agreed, visit FIC and get a new set ASAP. |
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Posted on: 2010/7/24 21:41
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The single biggest problem with communication is the illusion that it has taken place. - George Bernard Shaw Education is the best tool to overcome irrational fear. - me |
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| ChuckK | Re: 91 L98 "huffing " at idle. | ||
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Matt, Bogus,
OK, OK, how can I argue with two experts who are so convinced. I did run the fuel pump pressure test today : 43# at turn-on within 2 sec's 41# after electric pump stopped 41# held for 5 min's 35# with engine running. I know what to do. Just don't understand why it runs so good at all speeds and loads except for the pulsing idle. Thanks to everyone for all the help - - - CK |
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Posted on: 2010/7/25 1:26
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| TommyT-Bone | Re: 91 L98 "huffing " at idle. | ||
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Chair-man of the bored
 
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That's what happened to my 91. One bank went bad and the next year the other bank went bad. Replaced the whole batch with new and it was night and day difference. Keep us in the loop on this. Let us know your findings. |
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Posted on: 2010/7/25 1:53
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| Matatk | Re: 91 L98 "huffing " at idle. | ||
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Webmaster
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Fuel pressure numbers are within spec, but running might be on the low end. I'll have to check the FSM for the exact numbers. That might be attributed to the fuel injectors, possibly the pump. My 89 idles at 3 bar, 43.5 psi. |
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Posted on: 2010/7/25 11:41
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2002 EBM convertible, Magnusson supercharger, cam, headers, etc. 1989 Corvette...RIP |
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| ChuckK | Re: 91 L98 "huffing " at idle. | ||
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Guru Newb
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Well here is the result -- No Joy! Completed the install of the new injectors, O2 sensor, cap, rotor, plugs. checked fuel pressure - ok, cleaned and checked IAC - ok, cleaned and checked TB - ok. Checked all plugs/connectors - ok. Checked and found no vac leaks. No codes -
Idle is now at 1500rpm and STILL modulating - Miss at low load, ??????? now what ? CK |
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Posted on: 2010/8/6 16:19
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| Woodstock | Re: 91 L98 "huffing " at idle. | ||
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Well, I had someone, where a shorted injector killed the ECM.
The injector resistance on one was a lot lower though. |
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Posted on: 2010/8/6 16:53
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| ChuckK | Re: 91 L98 "huffing " at idle. | ||
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checked the old ones and the new ones - none shorted!
I do not have a VECI lable with the timing procedure and specs - can any one help? |
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Posted on: 2010/8/6 17:04
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| Matatk | Re: 91 L98 "huffing " at idle. | ||
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It is still possible the IAC is causing problems. Did you lube it with anything after you cleaned it? It might be time for a replacement. Anyway, here is the procedure for resetting the timing, IAC pintle, and base idle below.
Matthew Attach file:  KillebrewMinIdleSet.jpg (0.00 KB) |
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Posted on: 2010/8/7 11:38
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2002 EBM convertible, Magnusson supercharger, cam, headers, etc. 1989 Corvette...RIP |
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