joshwilson3

I've got an 89 with TPI. It was down a month as I did some things like replace spark plug wires. When I started it back up, I've noticed a high RPM flare up on start up. Both cold and warm start up. It shoots up to 1700, but sometimes to 2200 and takes a few seconds to drop down to idle. I don't recall it doing this previously. Around 3 times on a warm start up, when the RPM's dropped down. The RPM's hunted up and down from 700-1000 while still in park.

The odd thing is I notice no problems when it drive and driving around and idling. Just a high start up flare and a couple times a hunting idle before I put it in gear. I know my TBI car doesn't do this. The RPM's just go up to where it idles at. It doesn't shoot up, then go back down.

I've already had the throttle body off and cleaned up last year. TPS has been set and rechecked. New fuel filter, plugs, air filter, O2 sensor, 3-way check valve replaced last year. Timing checked. Just did plug wires. I recently put in a new IAC, and that didn't do anything.

Is there anything obvious to check for? Next few times I start it, I may just plug the brake booster line just to rule that out as a possible source for an air leak. I figure if I notice no change, then that should rule the brake booster out as I know those can be tricky to detect if they are leaking.

chevy1500z71

this happens to me all the time, my throttle body is worn out, it leeks air by the throttle shaft making the idle unpredictable. if i smack the throtle a few times it may decide to come down to idle. iv got a new throttle body on its way.

check and see if your throttle shaft has any play at all.

joshwilson3

How do you check for play? Though, you have to keep in mind that my idle is fine, and I have no high idle problem at all. I just have a high flare up for a few seconds when starting the car that drops down to regular idle while still in park. Though just 3 times the idle hunted after the RPM's dropped while still in park.

So, this is only a start up problem. And this problem came out of the blue as it didn't do this for years I had it. I just shut it down for a month, then all of a sudden it started doing it after I started it back up. So, I wouldn't think the throttle body would wear out sitting for a month. So, I'd think it would be elsewhere.

watajob

iTrader: (29)

You may wish to pull the IAC and make sure it's operating freely and the port is free of gunk.

joshwilson3

Throttle body was cleaned out a year ago. It only has maybe 1k miles since the cleaning. I already replaced the IAC and no change so I put the old one back in.

chevy1500z71

so you let the car sit for a month and now its acting up?before you go jumping to any conclusions, run that tank of gas out. i used to work at an atv shop, i cleaned alot of carburetors and i can tell you todays gas is only good for about month, it starts turning to jelly in the carb or throttle body in no time. im sure this isnt the case with every tank of gas you pump, but now a days its not uncommon at all to get a tank of gas that will be foul in a month or 2.

joshwilson3

I've done been through a new tank or two. But I don't think that would cause a high RPM flare up when starting the car. That would sound more like an IAC or something along those lines. But I've ruled the IAC out. I kinda wonder if it could be a MAF problem, but I hope not.

I would think of possibly it could be a vac leak. Something could of gotten bumped or what not. The only problem is I'd figure a vac leak would cause a problem all or most of the time instead of just for a few seconds when it starts up.

I think one thing I'll do is plug the brake booster line the next few times I start it to rule out a possible vac leak at the booster. As I know it can be hard to detect a leaking brake booster.

chevy1500z71

well if you think about it a vacuum leek will flare upon start up untill the ecm notices it and then compensates with the iac, so a vacuum leek is very possible. spray carb cleaner all over the throttle body and see if it responds to it, if it does that means its sucking the carb cleaner in through a vacuum leek somewhere.

also, do you use a k&n style filter? the oil from the filter can foul up with maf a biut over time, doubt its your problem but another thing to consider.

joshwilson3

I use a regular paper filter. I don't care for the K&N.

One thing I've read about checking for vacuum leaks, and looks like a real good idea. Is with the car off. You use a smoke machine. I think they use the booster line to force in the smoke. And that is supposed to fill up the intake and line that attach to it. With the smoke somewhat pressurized, it should find a spot to leak out if there is one.

Though, I'm sure that smoke may not get to every end of the lines, but I've heard people using it and finding leaks. I have heard of people using smoke in a can.

ASE doc

A smoke machine is a tech's best freind whne searching for vacuum leaks. And yes, the smoke will get through to any leak. On a leaking brake booster, expect to see smoke from under the dash.

One thing to be sure of though before you go too far is the ECM's idle learn. Youve performed a tune up and changed things a little. The car sits for long periods also. The ECM adapts to small changes and adjusts IAC steps to maintain correct idle speed under various conditions. It does this slowly and sudden changes in engine tune can cause the systems to act abnormally until it relearns.

You can help it catch up by performing the IAC relearn procedure. Basically, you need to warm up the engine, turn off the ignition, disconnect the IAC and restart the engine. Adjust base idle to 450rpm, reconnect the IAC. Be sure that all accessories are off for this procedure and that the idle screw plug is removed. If not you can remove it by drilling a hole in the plug and popping it out with a small screwdriver or awl.

joshwilson3

99% of the tune up stuff was done a year ago. Some was done closer to two years ago. The only recent tune up was replacing two air pump check valves as the passenger side one would make a noise on a warm start up, and I replaced the plug wires since I cut them while replacing the check valves.

I did do the IAC rest of start engine with RPM's raised for 5 seconds. Shut car off for at least 10 seconds. Then go drive around for 20+ miles.

I can't adjust base idle since I don't have a scanner. Though, I've set the rest like TPS. But I wouldn't think a base idle would all of a sudden cause a high start up rev. The plate part is missing so the PO had the base idle reset.

Would those party type smoke machines work? I've read of people using them, but others say they don't have enough PSI. And I've read of some making their own:



Looks like that first half you could just use an air compressor. Hook it up to a large metal can. Glow plug in the bottom which I've read to use an AC Delco 60G glow plug. Then run a line from the other side of the tank to the brake booster line. But I don't know how good the smoke would be compared to an actual smoke machine. If you can recommend a cheap smoke machine, that would be great.

ASE doc

My smoe machine cost $1,200. If you can figure out a way to make a cheap one, be my guest. Be careful though of pressure. Most smoke machines are regulated to only 10 or 15 psi max. My machine also uses baby oil for smoke. Another way to check for vacuum leaks is with pinch off pliers. Just pinch off any hose in question, look for an rpm drop. Is your alternator okay. Ive seen bad alternators cause high idle. Is the pcv system okay? The pcv valve can be a major vacuum leak. Is your coolant temp sensor reading normal? Is the fuel system lean for any reason. These are a few of the things I would be looking at with a fast idle, even just at startup.

Too bad you dont have a scan tool. That can tell you so much.

joshwilson3

What is the max PSI for the smoke machine? I think I read 2 or 5 PSI max on the homemade smoke machine. Don't wanna blow out seals. I could probably make one cheap with that glow plug as I already have an air compressor.

I had the alternator rebuilt by a local guy a few years ago. PCV valve has been replaced. Last I ohmed the coolant sensor it seemed to measure correctly warm and cold.

Though, the car idles just fine. It idles steady at 600 or 700 I think. The only noticable problem is the first 2-3 seconds after it starts. And maybe 2-3 times after the high rev went down, the idle hunted while in park. But when that happened, it was fine after I put it in gear.

I think the best thing is to get a scanner. Probably an Autoxray 3000. Just have to keep an eye out for one. The only problem with that model, is if you want to upload the data to your computer. That is another $50+.

joshwilson3

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ASE doc

Disconnect and plug the booster hose to verify but it sounds very much like a failed diaphram.

joshwilson3

Looks like that wasn't the problem as I plugged the booster line and it still started with a high RPM rev. Since now I've got to buy a booster instead of a scanner. I may not worry about it as it looks like more trouble than it is worth to figure this out.

ASE doc

Sorry for not getting back to you sooner. Once you replace the booster the IAC may relearn on its own and idle normally. This may take a few miles of driving though. You might also try resetting it again.

joshwilson3

I appreciate it. Though, I have to repeat. My idle is totally fine. It idles perfect. Only problem with idle is the 2-3 times the high rev came down and hunted while in P. But I kick it in drive soon after I start it, and from there it idles fine.

My problem is a high RPM rev on start up which only last 3 seconds maybe. So, its not an idle problem but a flare up on start up. Just about everyone seems to be confused and assumes I have an idle problem when that is not the case at all. Some will keep going trying to diagnose a high idle problem, when that isn't even happening. Just a flare. You crank it, RPM's shoot to 1700 or so then fall down to where they are supposed to be. I wait for the RPM's to drop before I put it in D.

So, that is why it is hard to figure out since it idles fine, just a flare up when I crank it.

ASE doc

I understand that you dont have an idle problem, only a flare after start up. Thats why I am telling you that if in fact the brake booster is a vacuum leak, once its replaced, the idle should return to normal. By normal I mean less flare on start up. Understand though that every computer controlled vehicle idles up to some extent on start up, before settling down to normal idle. This is simply the IAC openning to allow sufficient air for engine start, then closing down slowly to idle. Your engine's behavior may not be that abnormal.

joshwilson3

Thanks. I just wanted to check. As I had been talking to some other people somewhere else, and they would keep falling back to a high idle problem, when that isn't what was going on.

I'll replace the booster, hopefully it will be good since it is a rebuilt Cardone. And see what that does.

I'm pretty sure the RPM flare up at start up isn't supposed to go high like 1700 or 2200. As I don't recall it doing that before I shut it down for a month to do plug wire and some other things. As after I started driving it again, and I noticed it would hit in gear hard when I put it in D. I didn't know what was going on at first. But then noticed the RPM's were shooting up and I always put it in gear soon after starting it. So, I was putting it in gear with the RPM's high. I don't remember it doing this before the one month shut down. I was going to buy a scanner, but then the booster went.

ASE doc

1,300 to 1,500 is about normal with a return to idle at 550 in about 10 seconds when warm. A cold engine might go as high as 1,700 and take a little longer to come down. I have a 2003 Trailblazer in the shop and it goes to 1,500 on cold start and works its way down to 800 in about 15 seconds. Your familiarity with your own car is your best gauge of how it should operate.

joshwilson3

I appreciate it. I do recall it going to 2200 on start up warm or cold then falling back down. Though lately it seems to spike around 1700 on a warm start up. On my TBI 305 Firebird, there is no RPM spike. When I crank it cold or warm, the RPM's go straight to the idle RPM's. There is no spike, then RPM fall. Though that RPM flare on the L98 is pretty loud though like I hit the gas.

But I'm betting it is probably nothing to worry about. I've started an F150 a few times and noticed it goes to 1500 RPMs.

ASE doc

Any intake air leak will exacerbate the problem by allowing unmetered air into the engine, increasing idle speed. The IAC will have to compensate by closing further than normal. This will take a moment.

joshwilson3

I've got some data logged. I don't really know what to look for. But I uploaded some snap shots of the scan. For the last several weeks, I've noticed the cold RPM spikes aren't as high as they were. I do notice sometimes on the warm RPM spike, that at the peak, I can hear something from the exhaust. It's not a pop, so I wouldn't think it was a back fire.

I highlighted the engine RPM and engine run time.

This is a cold start up. I did the snap shot at the RPM peak right before it went back down to 950 rpm cold idle a few seconds later:



Here is a warm start up. I did the snap shot at the RPM peak right before it went back down to the 750 rpm warm idle a few seconds later:



This is another snap shot from the same warm start up scan posted above, but is 3 seconds later:

J91

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