: RPM Surge at startup



Spyder
01-03-05, 03:03 PM
Alright, I've spent the morning searching through and reading every thread that I could find with the words high rpm startup surge or idle in them, trying to find an answer to my latest problem.
Thus, after putting a new ISC motor on my 94 STS N*, on the first startup, the rpms ran at about 3000, so I shut it down, cleaned the throttle body, turned the screw in a bit and it was fine after that. Since then, about every other startup, it will surge to around two grand, then even back out at the proper idle. The ISC motor is good, there are no error codes, and it does it all the time, whether it is hot or cold. In reading one of the posts from bbob, he said to check fpr for a variety of symptoms, but in his instructions, it said to take the engine cover off and disconnect the vacuum line while it is running. Is this instruction for the newer engines, where the cover is not part of the intake or can I do this on mine? I also read a suggestion to spray a fine mist of water while running to find possible vacuum leaks, which I will try in a few minutes.
Also, on the first start of the day, when it does this quick shot of high rpm, there is a clicking sound that sounds like the lifters on my old lincoln used to. My guess is that there's not enough oil at them yet, as its cold and the engine has just started, which is why it only does it the first cold start of the day. It's not the cold carbon rap, as I flog my car on a fairly regular basis, but it does concern me a bit. Thanks a bunch!

haymaker
01-03-05, 03:40 PM
Spyder.. did you search using the words "idle speed control motor"?

Spyder
01-03-05, 03:41 PM
Alright...I now have the intake cover off, and the thin green gasket that runs around the outside of the top of the intake is missing a few pieces...I'm guessing that could cause a vacuum leak and higher rpms? I hope that is an obtainable piece from the parts store, and that I don't have to wait for it to be ordered and delivered...and I don't think that RTV sealant would be good in this case, as it could possibly be in contact with fuel. Time to ride the bike to the parts store and see if they have what I need... :suspense:

Spyder
01-03-05, 04:28 PM
Haymaker...Yea, all of this started shortly after I replaced the ISC...but I'll go search through those threads a bit too...

Myself and the parts store guys were unable to find any listing for this particular gasket in the books...when I came home and looked at it some more, it looks like it may have been an integral part of the cover over the fuel rail. Not sure what that piece is actually called, but it appears it may have been molded into the piece, rather than a gasket applied later? Anyways, I put the intake cover back on with the gaps in the green gasket still there, and it is back to the same as before. No worse, but no better. I would like to seal that up sometime, and it may still be the problem, but I'll have to do some searching to find the right part number to replace it with. In the meantime, I'll hope that someone on here will have more advice for me to try and figure out what the problem is.

Ranger
01-03-05, 06:36 PM
Spyder,
It sounds like you found your problem. If that gasket is missing a few pieces it surely will cause a vacuum leak. RTV might work if you lay a bead and let is start to set up. Then while it is still somewhat plyable, mold it to the correct shape. Let is set for 24 hrs. before putting the cover back on. A new gasket would obviously be the best solution. You may need to get that from the dealer or GMPD.

Regarding the FPR you are correct. DO NOT start your car with the cover off as it will run away unthrottled ie: WOT. That method with the engine running is for the later models with the one piece intake. In your case, just turn the key on to pressurize the fuel system (without starting) and look for fuel leaking out of the nipple on the FPR.

Spyder
01-03-05, 07:39 PM
What would that gasket piece be called? I can't find it at rockauto or anywhere else online... ?

haymaker
01-03-05, 07:56 PM
I would think if you have a vacuum leak it would cause the idle to be fast all the time unless the pcm changes the injector pulse or spark timing to slow the engine rpm. I would also think the pcm would try to bring the ISC back to its stop first to slow the engines rpm. I donít know how much the pcm can compensate for a vacuum leak?

I use WD40 to spray around the suspect area with the engine running and listen for a change in rpm. I use the plastic straw in the spray head so I can place the spray just where I want. This always worked for me on carbureted engines when locating a vacuum leak.
When you installed the ISC motor did you do the idle learn procedure or at least remove the battery cable?

Spyder
01-04-05, 05:22 PM
Alright...caddy dealer says that that gasket wouldn't cause a vacuum leak, and water mist sprayed around the edges caused no change in the running of the engine. The gasket is molded into the injector cover and would cost fifty bucks...I did get an idea this morning though...when I installed the ISC, I had to turn the plunger back a few turns from where it was to make things work right, or so I thought. ... ... Could this cause the plunger to overextend on startup, then as soon as the pcm realizes that the idle rpms are too high, for the isc to retract the plunger to the proper spot? This does seem like a possibility to me, any input?

Ranger
01-04-05, 09:25 PM
Alright...caddy dealer says that that gasket wouldn't cause a vacuum leak,
I must be misunderstanding what gasket you are talking about. I'm not that familiar with your year engine and I know it is different than mine.

Spyder
01-05-05, 01:49 AM
Yea...its a plastic piece that goes around the injectors and covers them up...they told me that it is only to seal out dust and grim from getting in there...makes sense, but I dunno...I'm gonna RTV'er up and see if that helps out any...

BeelzeBob
01-05-05, 07:45 PM
Someone must be misunderstanding you in the gasket discussion.

The upper intake is sealed to the lower by the plastic substraight gasket with a silicon bead around the perimeter. That bead certainly DOES seal the intake manifold vacuum and would cause a high idle or idle speed control problems if it is missing sections.

Unfortunately, that part is serviced only thru Cadillac and is probably $$$$.

I would use RTV to (sparingly) touch up the areas where the beaded seal is missing. You do NOT want to apply the RTV , try to mold it and let it set up before installing the cover. Apply a THIN bead of RTV where the seal needs repairing and install the cover immediately while the RTV is wet so it can adhere and seal. You need to wait for 24 hours or overnight at least for the RTV to cure so the engine vacuum will not suck it out of the joint as it will not seal immediately upon assembly.

You may have the idle speed control motor "adjusted" completely out of whack. With the engine idling, hold the throttle open to acheive about 2500 RPM. While holding the throttle open at the higher RPM depress the plunger of the idle speed control motor. It should retract and keep retracting until it is fully retracted. While it is fully retracted, disconnect the wire to the ISC motor to freeze it in position fully retracted. Allow the idle to return to normal by releasing the throttle lever. The engine should idle down to about 450-500 RPM at the closed bore position and there MUST be a GAP between the throttle lever and the plunger of the ISC. IF the engine will not idle down to the 450-500 then there is either something hlding the throttle open (cruise control link too tight...???) or something else. If there is no gap between the plunger and the throttle lever then screw the plunger in until there is about a .030 gap between the plunger and the throttle lever. This is a crude adjustment but it is good enough for who it is for...LOL. Seriously, you must get the ISC plunger to retract fully using the method described. There must be a .030 gap between the plunger and the lever. Too much gap and the ISC motor will rachet at cold starts and too little and the ISC cannot control properly. It must be able to retract the plunger clear of the lever and the engine MUST idle down to 450-500 when the ISC backs off completely.

Spyder
01-10-05, 04:11 PM
Alright...after adjusting the ISC plunger a bit, I've got about a dozen starts, and it seems to be doing much better...only goes to 1400 then immediately back down to around 7-800 for an idle. Tomorrow I'll do the RTV on the intake gastket, as I'll be out of town for a few days (Disneyland) and it'll have time to set.

burn2
01-10-05, 09:08 PM
I just replace my pistons, rods, and crank and bearings from a used engine. Striped out one timesert but i still went on. Not overheating yet, maybe it will run a few months. BUT.........RPM surge to 3k. Today I started her she was fine, ran for a half hour down to 700 rs just like she should then i gave her some gas idle went up and down from 3k to 2k to 3k had to shut her down. I don't know what's wrong.

Inspector
01-11-05, 05:58 AM
It is my understanding that it is pretty hard to hold a steady state no load off idle on electronically controlled fuel injection systems. Am I off base on this one? This could account for the surge. Mine does it but not when it is loaded as in gear.
Denny

BeelzeBob
01-11-05, 12:27 PM
It is my understanding that it is pretty hard to hold a steady state no load off idle on electronically controlled fuel injection systems. Am I off base on this one? This could account for the surge. Mine does it but not when it is loaded as in gear.
Denny


Well.....partially....

The modern engines with large intake manfold plenum volumes, long, tuned intake runners, large, single blade throttle bodies, etc....have a real tough time idling steady. Just the resonance in the intake manifolds (due to the tuning) will cause a roll or rhythmic up and down in RPM even if the throttle blade is fixed solidly still. The large , single blade throttle bodies make the engine very sensitive to throttle movement (the engine gets a lot of air at idle with a little tiny bit of blade movement). The idle speed control systems tend to "chase" the RPM and can actually cause an idle speed roll if the system gets out of phase.

All that being said......this stuff is theoretical and the type of thing that the developement engineers chase down and fix before the car ever is offered for sale. No engine would be released for sale with an idle instability problem so you really are not chasing some sort of inherent problem with the engine if it is not idling correctly...i.e...if the idle is unstable then something is wrong and it needs to be diagnosed and correct....the system does not need to be re-engineered. There are a lot of tools the calibrators use to control the inherent instability of the engine at idle with these systems. The spark advance is one of the main tools as the engine will respond instantly to a change in spark whereas it takes a finite amount of time for the engine to respond to a change in air flow (either from the idle speed control or the idle air control system)

So....I would agree that there is some reasoning behind what you say....but I would also say that it doesn't apply to trouble shooting a production car. Something to keep in mind but not something that should influence the trouble shooting necessarily....nor something to start re-designing the system for.

BeelzeBob
01-11-05, 12:30 PM
I just replace my pistons, rods, and crank and bearings from a used engine. Striped out one timesert but i still went on. Not overheating yet, maybe it will run a few months. BUT.........RPM surge to 3k. Today I started her she was fine, ran for a half hour down to 700 rs just like she should then i gave her some gas idle went up and down from 3k to 2k to 3k had to shut her down. I don't know what's wrong.


Sounds like the idle speed control motor is not operating correctly. Are you sure that nothing is restricting the throttle linkage and holding the throttle open or partially open....such as cruise control link or carpet under the gas pedal or ???

There is a lot of info on the idle speed control in the archives that may be helpful in diagnosing the problem. Search using the forum search feature in the tool bar above and type in "idle speed control motor" to search on.


You can quickly check one item that may shed some light on the problem. Enter into the onboard diagnostics system as normal by simultaneously pressing the OFF and WARMER buttons on the climate control panel while the engine is idling. After any codes display the system should prompt you with PCM? Answer yes to the PCM? prompt by pressing the HI button (HI is yes and LO is no) At this point the system will be in the PCM functions and the icons on the climate control panel will indicate the status of various engine functions. The one you want to focus on is the "LO" icon. The LO icon shows the status of the closed throttle switch. That is the switch that is in the idle speed control motor....the click or detent you feel when you depress the plunger of the idle speed control motor. Watch the status of the LO icon. If the throttle is closed (your foot is off the accelerator) then the closed throttle switch should be closed and the LO icon should be illuminated. Watch the LO icon to see if the closed throttle switch closes when you release the throttle. If the engine is surging like you describe and the throttle switch is not closed then something is restricting movement of the throttle or the throttle switch is not working (sticky contacts, dirty contacts, etc....replace idle speed control motor). Check it out.