Idle problems

[ernbuild]

Hi Guys I am having troubles with high Idle and surging at idle. Ive checked for vacumn leaks and cant find any. Any Ideas????
cheers Ernie :cheers:

[ZZZZZR1]

Hi Guys I am having troubles with high Idle and surging at idle. Ive checked for vacumn leaks and cant find any. Any Ideas????
cheers Ernie :cheers:

Have you hooked your car up to a Tech 1? You can adjust your idle with that down (if it isn't a vaccum leak or problem with the throttle body).

Have you done anything to the car recently? Did you powder coat the motor?

[tccrab]

Have you done anything to the car recently?

Any changes that you've recently made that could be at the root of the problem?

TomC
"Crabs"

[ernbuild]

Have powder coated the engine, while it was apart, because it had the same problem. Found a large vacumn pipe not connected under the phelnum, and when it went back together it was pretty good ,not perfect but a lot better. Seems to have gotten worse all of a sudden.
Have not had it on a tuner as they are hard to find out here.Only know of one guy that has the right gear. Have a haibeck chip in it, will that make any difference to the tuner.?? :cheers:

[tccrab]

It sounds as if you still have a vacuum leak.
I'd check all your vacuum connections including the MAP sensor hose and the PCV system.
It will make no difference to the tuner if you have a stock prom or a Haibeck prom.

TomC
"Crabs"

[tomtom72]

Just tossing this out as a fwiw.....on my 90 it wasn't one or two big (detectable) vac leaks. Rather, it was a collection of really small ones from all the rubber parts in the PCV sys and the dual connector at the bottom left of the T-body....just a thought.

I used small zip ties to correct the T-body & two larger Z-ties at the PCV holder & it's top connector. I replaced both 90* el's @ the I/H's and made sure to re-tq the plenum bolts after a run cycle. I know that was a lot of redneck engineering but alot of that rubber is NLA from GM. Even the new stuff I bought when it was available didn't last long at being tight. I guess our underhood temps must kill the rubber parts quickly.


:thumbsup:
Tom

[ernbuild]

Thanks guys, have tried most of those things, plus pcv's are new as is the double rubber thing they fit into. Have also clamped every bloody hose I can get at. Wil try the tune next and check again for leaks, ???:icon_scra

[ZZZZZR1]

Have powder coated the engine, while it was apart, because it had the same problem. Found a large vacumn pipe not connected under the phelnum, and when it went back together it was pretty good ,not perfect but a lot better. Seems to have gotten worse all of a sudden.
Have not had it on a tuner as they are hard to find out here.Only know of one guy that has the right gear. Have a haibeck chip in it, will that make any difference to the tuner.?? :cheers:

If you powder coated the throttle body and if they took apart the inside of it, that could be your problem. When my motor was powder coated, they took apart my TB and "adjusted" it. My car was running @ 2000 rpm @ idle. Using a tech 1 had no affect.... I spoke to Marc H and he said to send his TB to him and re would recalibrate it.

He did and fixed the issue! Problem is, I used a tech one to take the idle down and it did.... To where I had no idle! Had to trailer the car to a chevy station to raise the idle.

That "could" be your issue, if the shop took apart your TB.

Cheers

:thumbsup:

[tomtom72]

Ern,

Persuiant to what Dave just posted......if when cleaning the T/Body the "dag" was removed and/or compromised that will affect the sealing of the blades in the body's bores' and allow for an air leak to the extent that the IAC can not compensate.

"Dag" = blackish, rubbery substance forming a ring in the wall of the T/body bores to which the T/blades seat against. It allows for an air tight seal at 0% throttle.

Now I was thinking that if your idle issue is only after the motor gets hot, reaches closed loop operation? Then there might be a sensor issue, as in a 'soft' failure. O2's, TPS. IAC, MAP, IAT; back probing for voltage may reveal something in the absence of a scanner. The Emissions & Drivability Section in the FSM has diagnostic charts for voltage values. Or there could be a fuel delivery problem if the idle issue is in open loop & closed loop; including the pressure regulator (look for gas in the reg vac hose), and gas filter & pump socks & primary pump & injectors. I would also check the ing system as an outside source.

Okay, now the only other Q I have is the PROM. I'm not fluent in that stuff & I don't have a modified PROM from a tuner in my car. There was a TSB about the early 90 PROM concerning erratic hot idle issues.....only I Do Not Know if that is relavent here, sorry.:redface:......GM issued an updated PROM to solve those issues. The TSB is over at the NetRegistry site in the maintenance section....here's the link to the TSB.
http://www.zr1netregistry.com/bulletins/476503-1.jpg

Sorry I don't have anything better to offer.
:thumbsup:
Tom

Just thought of this, and I'm not trying to be a wise guy.....did you check for oil in the MAP hose & in the nipple at the MAP? That can impair the MAP's signals to the ECM.

[ernbuild]

Guys thanks for all the tech advice, but your talking to a builder here, not a mechanic, and whats worse is there are very few real mechanics out here that know anything about the LT5 motor. Its all a bit beyond me so Ill gather all this info and take it to the one bloke I know of that could help. The throttle body came off but was not powdercoated. I polished that myself , and I did know about the sealer inside. The problem only got worse recently, and I thought it may have been a poor batch of fuel, as it started to ping as well. Have a new lot of fuel now but havent had a chance to run it thru yet. Thanks again for the info, will let you know how I go. :cheers:

[tomtom72]

No problem Ern! I'm a building super, so I'm not a mechanic either!:sign10:

Okay, stupid Q: do you have the Helm Factory Service Manual set for your 90?? If not go here to see about getting a set.

http://www.helminc.com/helm/homepage.asp?Style=&mscsid=LLMSECDBX5GA8L6JS33K60N8W4KU4NV4

:thumbsup:
Tom

[ernbuild]

Dont have the Helm, but do have a set of factory manuals. Will they have the required info ?..:cheers:

[tomtom72]

Dont have the Helm, but do have a set of factory manuals. Will they have the required info ?..:cheers:

I would think that the FSM set that you have will have a section for "Emission's & Driveability". That is where the diagnostic flow charts and aids to diagnostics are in the Helm set. I would guess that most of the other manuals like Motors & Chilton's...etc are mostly laid out the same way, I would suspect??

:thumbsup:
Tom

[ernbuild]

Thanks Tom have a bit of time today and hope to check a few of these items out. However the high idle is right from start up ,not once it warms up. What ever is the problem has no effect on performance once moving along, but can be a Pain in the a#SE IN TRAFFIC. :cheers:

[tomtom72]

Thanks Tom have a bit of time today and hope to check a few of these items out. However the high idle is right from start up ,not once it warms up. What ever is the problem has no effect on performance once moving along, but can be a Pain in the a#SE IN TRAFFIC. :cheers:

Ahhhhh Ha!!!! Now I have a sympton that I can use as a starting point, thanks Ern!:thumbsup:

Okay, go get a very rigid 3-ring loose leaf paper binder. Then remove the air duct from the air horn. Start the car. Take the binder and place it over the air horn, Hang On To It really tightly as the LT5 will suck it in if given 1/2 a chance....our motors like to move that much air. If the motors stalls out quickly then you do not have any vacuum leaks past the point of the binder. That's good. Then go look at the Dowel diagnostic down below for testing the 2* t-blades to see if they are seated correctly.

If the motor does not stall out but looses some rpm then you have an alternate air source. Good luck as they can be a right PIA to track down if it's a collection of small ones. Just need to make sure that the binder seals well against the air horn.

You can do this in addition to the above scientific diagnostic procedure.....:sign10:......with the air duct removed and the motor at idle use a soft wood dowel, 1/4" dia., to push against the 2* t-blades. If the idle goes down then that "Dag" that I spoke of has been compromised. You can maybe do the same with the 1* t-blade, but ya need a really small dowel as that blade is 19(?) mm. That is to say that the vacuum leak is really an air leak thru the 1* &/or 2* t-blades at the "Dag". Now if the trick with the 2* blades has no affect on idle speed, and the loose leaf binder said you got no air(vacuum) leaks; then you have to suspect the IAC or/and the primary T-blade. The 1* t-blade has an adjustment screw to check and I'm sure it must have a "Dag" ring also. Now confession time: the idle screw I have no idea where & what to do....and the only way I know to check the "Dag" s to dismount the T-body and look for it from the back side of the blades. You maybe be able to see it with the motor off and all the blades at WOT position. Oh, also check the TPS for correct voltage, 0.54 volts @ 0% throttle (throttle blades closed), motor running at idle.

I hope that this may give you & your mates a direction to start from. Also, with the loose leaf binder....hold on tightly and expect that after about 30 seconds or sooner it will start to fold up and get sucked down the air horn! This ain't like the old days of carbs when you could cup your hands over the air horn or use a shop rag to cut the air flow to check for a vac leak.....our LT5's will ingest that stuff. All this BS with dowels & binders is due to the fact that a scan will not show any of these t-blade issues in the scan results, the only thing a scan tells ya when ya have a vac leak is that the MAP voltage is wrong.

:cheers:
Tom

[ernbuild]

Thanks Tom will try that today. Missus way/layed me yesterday and ended up sanding the deck and sealing it. Will let you know how I go. Ern.:cheers:

[ernbuild]

Well tried a couple of things. First up could not get the engine to stall by blocking the throttles off as you suggested. In fact it made very little difference to the revs. Tried a few things to cut off air flow, including a rubber sheet a quarter inch thick. However was able to lower the idle by the adjusting screw right at the back of the mechanism that the cables attach to. Damned awkward, 8 mm nut on threaded rod with an allen key on the end to turn the screw. Managed to get it down to 900 to 1000rpm, but its still surging about 200 rpm. Its a lot better than it was but still has a vac leak by the sound of it. What do you reckon?? cheers Ern. :cheers:

[Jeffvette]

Well tried a couple of things. First up could not get the engine to stall by blocking the throttles off as you suggested. In fact it made very little difference to the revs. Tried a few things to cut off air flow, including a rubber sheet a quarter inch thick. However was able to lower the idle by the adjusting screw right at the back of the mechanism that the cables attach to. Damned awkward, 8 mm nut on threaded rod with an allen key on the end to turn the screw. Managed to get it down to 900 to 1000rpm, but its still surging about 200 rpm. Its a lot better than it was but still has a vac leak by the sound of it. What do you reckon?? cheers Ern. :cheers:


You are masking the issue when you adjust the set screw. That set screw was set by Rochester and MM never touched them when they assembled the motors.

By completely closing off the air intake, you eliminated any possibility that the TB was the issue. My guess is you have some leaks in around the fuel injectors at the injector housing.

[tomtom72]

Jeffvette = The West Coast LT5 Tuner, Sensei Master....:worship:

So it's "what he said, man!". No BS.

Did you have the injectors out? Seems like the prime suspect would be the large O-rings that are inside the I/H injector ports. In any event the air isn't coming from the T-body.

:cheers:
grasshopper

[ernbuild]

Yes Guys the injectors were out, however all new O rings when it went back together. By the way the engine was apart 18 months ago, and was running ok till a few weeks ago. Have monster headers fitted, and relocated the O2 sensors further away from the engine. Also no cats, and burns a little oil at start up. Will check O2 sensors as well, as read also if they have a build up of gunk can cause surging. Will keep you advised. Thanks again for all the info guys, cheers Ern. :cheers:

[Jeffvette]

Yes Guys the injectors were out, however all new O rings when it went back together. By the way the engine was apart 18 months ago, and was running ok till a few weeks ago. Have monster headers fitted, and relocated the O2 sensors further away from the engine. Also no cats, and burns a little oil at start up. Will check O2 sensors as well, as read also if they have a build up of gunk can cause surging. Will keep you advised. Thanks again for all the info guys, cheers Ern. :cheers:


You reuse the factory injectors or replace them with something else?

Your O2's are not the problem, as they do not come online until after the car is warm. You say you are experiencing this issue on start up as well.

[ernbuild]

Reused the original factory injectors, but replaced the O rings as I said, however the vette only has 20000miles on the clock. I didnt give much thought to replacing them. ??????:dontknow:

[tomtom72]

Ern, yea with 2k on the clock you would not think about the injectors. If they are OEM then they are MultiTechs (ACDelco) and are common steel.

Also, if they were exposed to gas made with methanol, over time they will have service issues due to the methanol. Over here we have had methanol for yrs mixed in our gas. Since 93 at least and on cars made before the 93 mod yr the injectors were not able to handle exposure to methanol for any long periods of time without suffering: metal corrosion & coil failures from shorts. A scan might tell you it's the injectors as you can look at the fuel numbers real time & see what's going on.

As an aside, the OEM units in my 90 failed at about 10k miles, but the car came to me with 7400 miles on it. It sat alot before me, and as soon as I started driving it I got about 5 months down the road and the coils went out due to the methanol. The metal was none too great inside the nozzel either. Mine failed without warning, fine one minute and a miss the next as mine had coil failure.

:cheers:
Tom