I have a 90 vert with the L98. It has nearly 200K on the clock thought the motor was rebuilt and runs better than great, most of the time. It has a stumble just off idle. I've been pursuing it as being related to the EGR or IAC. It's none of that, nor injectors. I have the receipts to prove it. My FSM suggests low voltage as a cause of low idle/just off idle throttle response issues. The battery is fairly new. What about my alternator, can I be getting low output at low RPMs? If I am, wouldn't the battery pick up the slack? How can I prove it WO buying an alternator? Thanks in advance.

 

If you voltmeter increases when you rev the engine....chances are you might have some of those crappy pulleys on the engine. At idle...the alternator is barely charging....but when revved up..it does fine. I would look into that...or have your alternator removed....which is not a big deal and have it tested.

Your battery is not some miraculous component that can store volts and amps higher than it is designed to do. If you alternator is not doing its job...you will eat up your battery's power. That is why your alternator has to charge higher than your battery's 12 volts to keep this system working correctly.

Also...having receipts are great....but it is installing part that is worthy of installing is the most important part. NOT all parts are equal.

Have you verified your fuel pressure???
Checked your setting of your TPS????
Check your integrator value when running????
Is this problem occurring when it is in OPEN LOOP and CLOSED LOOP??? OR.... only one of the above.

DUB

 

A stumble off idle can be caused by a vacuum leak.

 

Tell me more about the "crappy pulleys". I assumed these were OEM. The FSM suggests that my symptoms are related to low voltage. I thought a worn out alt might be the problem.

 

All new vac lines. It's been in my mind I missed something but it has all been replaced. EGR unhooked makes no difference too. I've considered fuel pressure, as it starts harder after having replaced the injectors and FP reg. It's not got me dead in the water, it just bugs me.

 

So with a fully charged battery, what voltage do you see at:
Battery with the engine off key off and with engine off key on?
Battery with engine running at idle?
Alternator with engine running at idle? At 1500 RPM?

 

I'll have to check. I work 12s and it might be a few days before I can check it.

 

If you can...measure the diameter of the alternator pulley and crank pulley. If the alternator pulley is LARGER than factory....then it is turning SLOWER...thus NOT charging when at idle.

I would hook a scanner to your car and see what the INTEGRATOR value is. The reason is this. Getting the tuned runners and plenum to perfectly seal and NOT have a vacuum leak is a bit tricky...ESPECIALLY if when you had it off when changing your injectors...SOME BOZO got on the machined surfaces of these parts with a brisket pad and damaged them. I run into this quite more often now than back in the day when the cars where new. I take my stethoscope and listen if I see my INTEGRATOR value is higher than I like. I sometimes have to do it a few times to get the intake runners and plenum sealed up and NO vacuum leaks that I can hear or INTEGRATOR value that is NOT correct...which should be about 128 at idle...in perfect world. If you a have Factory Service Manual...the INTEGRATOR is described a little bit in the trouble codes for your Oxygen sensor(s)...and if my memory serves me correctly....You have a MAP sensor...and its voltage can show up being a bit high. I think ( if memory serves me correctly again) should be about 1.4 volts at idle.

DUB

 

Any idea about the diameter of stock? That's an easy check. As for the rest of what you said, I'm a bit lost. I'd love to find a good GM guy around home who knows this stuff and turn him loose. I think I'm going to look into a tuning tool for this thing. I didn't want to spend a bunch on a tool I'd rarely use but I may be down to it if I want to keep this thing a while. And, I do.

 

You can find a good used Snap-On scanner . I think it is a MT2500 "red brick". They are inexpensive because they are out dated...due to the computers today require much better capabilities in a scanner due to all that eh computers in cars today can perform....BUT ...this type of scanner is useful for you to obtain data when running.

No idea of stock diameter....I am waiting on you to give me measurements. Or go to your local parts store and measure on there...and compare to yours. Also the part number on your serpentine belt is still visible would be different than a correct stock belt due to the pulleys causes the belt length to change.

DUB

 

OK. Checked it tonight. Looks like 2 1/4". (poor lighting, metric/US measuring tape, and bifocals, sucks to be an old fart)

 

sure your timing/tps is correct? with you on the old fart...

 

New, TPS, IAC, injectors, other things I can't call to memory right now. First was plugs, wires, ign stuff. Then vac hoses. I changed everything that was not to pricy to when I started chasing this rabbit. (Car was old, why not.) Then I moved on to pricy stuff. At almost 200K I've wondered if my TB might just be leaking enough at very low speeds to cause this. I've wondered about fuel pressure being low. The FSM suggested low voltage and that brings me here. I hate to just keep throwing money at it but where I am now, all the suspect problems are costly. I don't trust the wiring as a PO as "fixed stuff" a few times, so that's always in the back of my mind. I will get this. God willing.

 

I been chasing a similar problem and as it hesitates, fuel pressure drops for a second as effect happens. Not to discuss my problem here and what I have done so far, but my future plan is to replace the fuel filter which I don’t think will cure it. But then I am going into the fuel tank and pull the pump. I want to get a good look at the sock, bottom of tank and anything that does not look as I think it should.

Just a thought for you.

 

By all means, jump right in. My thought has been fuel pump too, as the FSM suggests it as a possibility too. I'm just trying to nail it down a little before I start throwing money at it again. With nearly 200K miles showing, I'm sure the rebuilt motor didn't include the FP and alt. I'd bet they were simply bolted back on. Time and money, never have them together.

 

Through this entire post...you have not stated the voltage READING when it is idling.....which makes a BIG DIFFERENCE. What is the voltage and at what RPM???

Also numerous questions have been asked that can aid you and NO RESPONSE in these questions????

I asked you to give your fuel pressure in post #2.

Integrator value I asked in post #2 would coincide with Cliffs reply in post #3

I asked for your TPS reading in post #2...and nothing....but yet others reply with requesting you check this also.

If you are one of 'those people' who want to fix their car and NOT have the correct tools to AID you in doing so....it gets really tiring to get the problem resolved without important information. You ASKED for help....TRYING to help....BUT getting NOWHERE fast. 'These cars' do require tools and the know how to get them running and keep them running. And YOU can do it...but not if you are not prepared and willing to give reply's to important questions asked.....and obtaining the tools for gathering this important information.

Maybe others who have replied to your post do this fora living also...that I do not know for certain.....but I do this for a living daily and I am trying to help. For free. But I will loose interest if you keep wondering what is wrong ...but yet have not replied to questions. Kinda ridiculous isn't it.

DUB

 

You are right on the money, I don't have the tools. Honestly, I don't even know what an Intergrator is. I know you are trying to help, and I greatly appreciate it. I think you mentioned a Snap-on 2500. I'm looking for one. I doubt I'll be able to use or understand it, but I can learn. If you lived near me, I'd gladly bring my car to you, turn you loose, and pay you well when you were done. You don't. I can't find anybody close who does. Around here, almost nobody is into these cars and those who are don't have a 90. The other rub for me is time. I work 12s and lots of OT. Other things simply come first. You're right. I was trying to fix it by throwing parts at it. Not the best thing but kinda my only option. If you know of anybody in the Parkersburg/Marietta area, I'd love to take the car to them. I didn't mean to waste your time or **** you off. I appreciate your input and understand how you feel.

 

keep posting, someone will help you....it's better if you work on one issue at a time and be specific with the details....DUB or someone else will gladly help you....that's why we are all here....get a fuel guage for starters...we always need to know fuel pressures when you have a problem....you stated you have a new tps...but is is set right....timing could be your issue too....timing lights are not expensive....if you're going to drive a c4, eventually you're going to be a gearhead.....good luck, keep posting..

 

First off...you are not "pissing me off"...I am still willing to do all I can to help you...and if that means explaining how to use a scanner and read the data...that is fine also. But also with that written...IF you are someone who does not know this 'stuff'...would it not be in your best interests to give the information requested that people ask for??? I do not mind waiting for it due to your work schedule...but not replying to it and possibly thinking that it means nothing is something else entirely...don't you agree?? Moving on.

The term 'integrator' is a term that you will see on the screen of the scanner...and it is mentioned in the service manual. In a 'perfect world' your engine should be running at an integrator value of 128. This value is outlined in the trouble code 16 for your Oxygen sensors. And I sue this value to let me know if there is a vacuum leak AFTER the throttle body...where your IAC is located which is controlling the idle and and leaks AFTER IT.

TRUST ME...I have had more customers than I can count that threw EVERY part at their engine and only to find out that NONE of them helped. And often times installing a less expensive NON-GM/ACDelco parts have been a problem also....in my experiences. This is where trying to fix something without the knowledge and the tools to give you the information needed. SO...unless you want to waste a lot of your hard earned money...do as you wish....you might get lucky and fix it.

If you have a FACTORY Service Manual...you might need to...when you can find time...is to do some reading to familiarize yourself with some of the information.

I am still here....but I need information. Without it I am "FLYING BLIND' and can not offer anything else.

Keep in mind that this car really needs good power to keep it running right. So this issue with your alternator would be TOP on my list to get fixed.

DUB

 

In my opinion you need certain basic tools to diagnose a problem like this (and many others):

1 - fuel pressure gauge
2 - timing light
3 - digital voltmeter

If you are missing any of those it will be much more difficult to troubleshoot problems with your car.

An ALDL (Assembly Line Diagnostic Link) cable and scanning software for a computer would also be helpful. A dedicated scan tool can also be used, but that's less flexible and doesn't allow data logging.