Hunting Idle, hesitation and stumbling while driving and no SES light

Hi guys,

I am once again looking for some adivice with an issue that has me with no answers. Last time this forum was great and helped me through some issues. My Riv was running great before today. It is a 96 Supercharged with 210,000 km

Issue
The car hunts for a stable idle rpm between 500 to 900 rpm in both Park and Drive. Sometimes it smooths out for a minute or so.
While driving at a steady 50 to 55mph the car hesitates and shakes (sometimes it smooths out). Today when i stopped for a school bus the car stalled. It took quite a few cranks to get it to go. However, it still starts on first turn of key. I hooked it up to a scanner. Scanner found no codes (SES light is not on anyways) But the scanner did pick up random misfires on all cylinders, (no pattern that I can detect,) but I guess not enough to get the SES light to come on. I drove home 45 minutes and still did not set off SES light.

What I have done.
Last 5000 kms replaced the following: S/C coupler, plugs, wires, filter, fuel filter, put a few used injectors in (to fix previous idle issue),
Today: cleaned MAF sensor, cleaned IAC, cleaned Throttle body, took off coils and wiped down ICM with dialetic grease along with any other connections, checked vaccum lines

I am at a standstill right now. Without any codes i am not sure as to what is causing this issue, Any help would be appreciated.


Thanks,
Rod

Hows the quality of gas? Whats your fuel rail pressure look like? Are the used injectors failing?

Gas is good, 91 octane. I don't beleive it to be an injector/fuel issue because of the erratic nature of the misfires and also last time I had an injector issue it was constant and threw a code. However, i could be wrong. Looking at getting a fuel pressure tester thought, just incase.

Sounds to me like a dirty MAF sensor. It's worth a try... but cleaning it is hard. You pretty much need to remove the MAF to get it out, which requires a security torx (e- socket) on some models. Not sure if the '96 had the security torx heads or not. Once you pull the sensor out spray it down with MAF cleaner, a specific cleaning product. Do NOT spray it with carb cleaner or brake cleaner or you will be looking for a new sensor.

Do you have a K&N filter? If you over-oil the filter it will interfere with the MAF sensor.

Derek,

I thought along similar lines and pulled the MAF and gave it a good cleaning with MAF cleaner, but it still acted the same. Quick question, would an inoperative MAF set off the SES?

Thanks,

Rod

I would think that if the MAF were that messed up, it would set a light (P0101). But, it wouldn't be the first time if it doesn't......

I am wondering if you have a vac leak.

I am wondering if you pulled the TB to clean it. (if not, this could be a big part of the problem)

I am wondering if your replacement injectors failed quickly, and if you might want to get new/rebuilts from rockauto.com.

I am wondering if you have checked the wiring to the computer under the coil pack.

Finally, I am wondering if you have a failing temperature sensor that is flipping between telling the PCM that the car is cold (raising the idle) and warm (reducing the idle). I'd say probably not because if that was the case I doubt you'd see the problems at cruise.

I very strongly suspect you've changed something else that you're not saying because you don't think it's important. For instance, did you put it a K&N air filter? What other mods does the car have?

AlbertJ,

Thanks for the reply. I have checked/cleaned all of the items on your list. I even hooked it up to an automotive scope and checked the coil pack, injector functioning and so forth. The one thing that i did notice on the scope was cylinder one coil was lower in volts than the rest. This does make sence as the random misfiring I was having was occuring most frequently on cylinder one. Something I will have to remedy.

I do not have any mods nor a K and N filter. Just a stock machine. But for whatever reason, the car is now running fine, no more random misfiring. The idle is fine and is presently not hunting. I am not sure if it cleared itself, or if all the cleaning and checking I did fixed the problem. I forgot to mention I cleaned the EGR valve (even though the scanner showed it to be opening and closing properly/right time).

This acting up has me puzzled. Glad its running good, however, I just don't like not knowing what was the cause of the problem. When i was reviewing the scanner information, I felt that the O2 senors was not switching from lean to rich fast enough (even through the SES light did not come on). But I don't think they fix temselves (checked wiring of the O2 sensor and it fine).

Thanks guys for all the tips and advice.

Rod

Rodman wrote:

AlbertJ,

Thanks for the reply. I have checked/cleaned all of the items on your list. I even hooked it up to an automotive scope and checked the coil pack, injector functioning and so forth. The one thing that i did notice on the scope was cylinder one coil was lower in volts than the rest. This does make sence as the random misfiring I was having was occuring most frequently on cylinder one. Something I will have to remedy.

I do not have any mods nor a K and N filter. Just a stock machine. But for whatever reason, the car is now running fine, no more random misfiring. The idle is fine and is presently not hunting. I am not sure if it cleared itself, or if all the cleaning and checking I did fixed the problem. I forgot to mention I cleaned the EGR valve (even though the scanner showed it to be opening and closing properly/right time).

This acting up has me puzzled. Glad its running good, however, I just don't like not knowing what was the cause of the problem. When i was reviewing the scanner information, I felt that the O2 senors was not switching from lean to rich fast enough (even through the SES light did not come on). But I don't think they fix temselves (checked wiring of the O2 sensor and it fine).

Thanks guys for all the tips and advice.

Rod

This is a long shot -- but -- when you were doing all that checking, did your scanner show the status of the crank sensor?

Crank sensor failure, it's a weird one. When it fails typically it will not set a code and it fails funny. In the 2 that I have replaced, the failure pattern was it would quit warm. That is you'd start and drive a while, park the car, and not be able to restart until it was fully cool. The second time it quit similarly - however at that point I had found out that the replacement ones did not last but so long. I put a factory one in and it's been fine since and should stay that way. The way to verify is to remove it (it is behind harmonic balancer) and warm it with a heat gun or by dipping the plastic part in boiling water while reading with ohmmeter. It will either open (resistance goes infinite) or short when warm (I forget which ATM). Your failure does not sound liek the crank sensor but it crossed my mind - and it's one of the failures that's hard to check and often will not set a code. If you're over 80K miles or so it might become an issue. If you do replace it get a good one (a Delphi for instance) and odds are it won't fail again.

And the cleaning and checking may well have fixed thngs for now especially if you cleaned the (many) engine grounds, including the ground plane under the coils.

You may want to think about replacing the cyl 1 coil; if you get 2 junkyard coils from other similar year GM SC 3800s you'll get 1 or 2 good ones, just measure them both and pick one to install. In general they are tough little buggers. When I lost a coil, it was due to a bad ignition (spark plug) wire. I think it was the cyl 1&4 coil.

Another hard-to-spot failure would be the ignition switch.

Your further thoughts?

Albertj

Hey guys,
I guess i didn't knock on wood last night when I typed my responce. The riv was acting up again today. Same issue! This time I have found a clue. Rain/water. Last time she acted up it was raining quite hard, as it is today (Not sure why I didn't consider this before). Last time it started idling weird and stumbling about half an hour into my drive and continued to do so. This time five minutes into my drive she was acting up. It was once again raining hard. When the rain started to let up, the car began to drive better. I have owned this car for only a few months, so this may have been an ongoing issue with the PO, or it may of just started. Still no SES light, I guess the misfiring is not consistent enough to set of a PO300.

AlbertJ,
I had been thinking about the crank sensor too. The scanner did not show the crank sensor information. I will probably replace it anyways (always nicer to replace on your own schedule). I am going to the yard to pick up a couple of coils soon.

If anyone knows the best way to detect where the water is causing the problem, I would like to know. I will trace all exposed wires looking for fraying and weakness. But is there a simpler way? I am new to fuel injection. The last twelve years I have driven carbed cars (regals/f-100's) so I have never had a similar issue/complexity. Has anyone had this type of an issue before that they could share, so I can create a likely suspect list?

Thanks guys,
Rod

Get a spray bottle full of water. Start the car. With the engine running, start spraying the plug wires one by one, then the coils and the module. Cover all the ignition components until you find the misfire.

Also check the bundle of wires at the big firewall connector. The ones in the corners on some cars have a habit of fraying/breaking. Not sure why. Time consuming but not difficult to find and fix.

Albertj

Abaddon, AlbertJ,

I once again have moisture here, lots of it (nearly 3ft of snow) and the idle stumbling is back. I haven't driven much in the last two weeks as I carpool to work.

As you fella's suggested, I went through the wiring (from ICM and also at the suggested firewall location) and fixed the issues that I saw. I also swapped out coils and ICM for a used replacement. I went through and misted everything with water to try to recreate the issue, but to no avail.

Just abit of a refresher:
Idle is occassionally stumbling, dips to 550 and will go up to 1000. The maddening part is the occassional nature. It has also seemed to correspond to moisture and will only do it when car is at operating temp. I have new plugs, wires (AC delco) and cleaned/checked all of the traditional culprits. The car when rpm stumbles, lightly misfires (on scanner) but does not set off MIL light (MIL light does work).

Here's my question:
I have just noticed that the intake manifold gasket is leaking (260,000Kms on engine). Coolant is pooling on top of the intake manifold in one of the crevices. I will replace intake manifold gasket next week. Although I have noticed a relation of idle hunting, stumbling and highway speed hesitation (enough to vibrate car around 50mph) to wet weather, could the leak in the intake manifold be responsible for my problems?

Thanks for any help board members,

A frustrated Buick guy: Rod

im not much of a mechanic but a leaky IM on a s/c car can be a culprit.yes!

robotennis61 wrote:

im not much of a mechanic but a leaky IM on a s/c car can be a culprit.yes!

The aftermarket and revised GM I/M gaskets are better, once you do this it won't likely happen again so the sooner you fix it the happier you'll be.

I/M might also be cracked, leaking air in on the vacuum side. Result would be the ECU's settings are bogus because the air/fuel mix is wrong no matter what the ECU sets it to esp.at high vacuum.

Absolutely have to replace the i/m, why not find a good used one? New offa RockAuto.com the lower intake manifold is about $300.00 - thing of it is, there were a lot of these engines made, I suspect if it turns out you need one I'd contact moradpartscompany.com, Ed Morad specializes in the 3800 engine and should have/know where to get one without taking out a mortgage...

As for your frustration - most folks have found that these cars used usually come with a load of issues that you have to isolate and fix one by one - and having fixed the issues they are then a dream to drive. Swift and smooth.

Albertj

albertj,

I'm figuring/hoping it to just be a bad gasket. I have coolant pooling in the Lower intake manifold right below where the supercharger nose cone bolts to the main S/C body.

Are craked intake manifolds common on S/C 3.8's?

Thanks for the help guys,

Rod

nope. if your original gasket is the crappy paper like gasket that was on my 95,it probably just blew out. rip the S/C off and take a look before you start driving yourself crazy. a crack in an aluminum manifold is easy to spot.i bought my gasket local for around $15 bucks. line up a used manifold from a junker just in case. a used manifold from a local pick a part in my area will run $25 or so...