Diagnostic Help

Status
Not open for further replies.
Joined
Apr 20, 2013
Messages
128
Location
-
2008 Saturn Aura
130,000 miles

GM 3.5L High Value V6 - Pushrod w/ VVT
GM 4T45E transmission

I really like this car; looks good, underrated performance, and drives well. At least, it used to... Lately I've been having some issues which I would like to fix the right way without just throwing parts at it. Car doesn't have any codes stored and no CEL. Still drives, but definitely has some problems. If it's a minor fix, I'm definitely willing to keep the car going, but something major and I'll just defer the decision to repair until something actually breaks for real or starts throwing codes.

Symptoms:

1) Hard shifting. 2-3 is the worst, 1-2 and 3-4 can be bad too. This seems to point me to typical GM 4T45e transmission woes. I'm not opposed to eventually putting a reman transmission in the car, but I'm not sure that's actually the problem with the car because of the other symptoms.

2) Occasional hesitation and poor acceleration. Pretty straightforward here, car sometimes doesn't like to get up and go. Other times, press the pedal and it feels like when it was a brand new quick V6 with nothing wrong. I'm thinking this could be MAF or MAP sensor. MAF was cleaned 6 months ago with MAF cleaner.

3) RPM fluctuations. This makes the car really annoying to drive. RPMs like to move around and make the car feel like its lugging and jerking. Even on the highway sometimes the RPMs will move around like the torque converter isn't locked, but then it will "lock up" and RPMs will stay still. Torque converter/transmission problem?

What I know:

Throttle body was cleaned a year ago which fixed a rough idle.

MAF sensor was cleaned 6 months ago with MAF cleaner. It was a long shot attempt to fix some of the symptoms I described without throwing parts at the car.

Engine Vacuum doesn't seem right. Using Torque app I see that my vacuum is around 18 "Hg at idle but drops to around 5 "Hg when driving...and rarely comes back up unless I fully take my foot off the throttle. Normally, I thought the vacuum was supposed to come back up after the initial drop upon acceleration as everything "evens out." This might be a slightly clogged exhaust?

-----

Any thoughts on what may be causing these problems? I don't have a ton of time right now to be troubleshooting and fixing things, but I would like to know what is going on so I can get around to fixing it if possible. Like I mentioned, car still drives and doesn't have a CEL, so I'm not panicked about any of these symptoms yet. I'm moderately handy, can do routine repairs and even some moderately complex ones if you give me a good set of detailed instructions. If it's transmission, I'll be taking it to a transmission shop though.
 
If no CEL, then each of the different sensors need to be hooked to a scope to examine wave forms; especially the MAP and MAF sensors.
 
That's what is frustrating. Car definitely does not run like it should, yet it isn't throwing any codes.
 
The offending sensor(s) are working fine most of the time to keep the computer satisfied, but then act up enough to cause drivability problems. Happens all the time to different cars. You either hook them up to a scope or wait for them to get bad enough to throw a code.
 
From what you are describing , possible cause could be the throttle body assembly. The throttle position sensor is integrated in this unit. I would perform a capacitor discharge on the battery. Then perform a throttle body relearn procedure. Unfortunately I think this is going to require a fully bi-directional scanner with scope function to properly diag.
 
Originally Posted By: VetteElite


Symptoms:
1) Hard shifting. 2-3 is the worst, 1-2 and 3-4 can be bad too.
2) Other times, press the pedal and it feels like when it was a brand new quick V6 with nothing wrong.
3) RPM fluctuations.


1) Pressure control solenoid? That's what it sounds like

2) & 3) - you've got a bad ground somewhere. Add some bonding straps and see if the problem goes away. These new computerized cars love having good grounds.
 
Originally Posted By: VetteElite

Engine Vacuum doesn't seem right. Using Torque app I see that my vacuum is around 18 "Hg at idle but drops to around 5 "Hg when driving...and rarely comes back up unless I fully take my foot off the throttle. Normally, I thought the vacuum was supposed to come back up after the initial drop upon acceleration as everything "evens out." This might be a slightly clogged exhaust?
Any thoughts on what may be causing these problems?


Transmission shifting issues are routinely misdiagnosed due to engine performance issues, engine doesnt run right the transmission doesn't run/shift right either its really that simple. It looks you might be dealing with a partially clogged cat, that will cause all you describe without throwing a code until it gets so bad it causes a misfire and flashing CEL.
Hook up a mechanical vacuum gauge to the engine, to insure proper readings, if its indicating a bad cat its time to go a bit further down that road.

http://www.aa1car.com/library/converter.htm
 
sounds more like a clogged cat to me.
disconnect the pipe in front of it and look at the vacuum again.with a real vacuum gauge.
do a before and after.
if the map is lying to the pcm same result.transmission is being programmed to shift with the data the pcm sees.
if engine is down on power and the shift is being performed for the power it is supposed to have under said condition it will seem harsh.
 
not even close.if that was sarcasm you are wrong.
its a saturn in name only.
Originally Posted By: Donald
Easy to diagnose the problem. It's a Saturn.
 
Originally Posted By: Trav
Originally Posted By: VetteElite

Engine Vacuum doesn't seem right. Using Torque app I see that my vacuum is around 18 "Hg at idle but drops to around 5 "Hg when driving...and rarely comes back up unless I fully take my foot off the throttle. Normally, I thought the vacuum was supposed to come back up after the initial drop upon acceleration as everything "evens out." This might be a slightly clogged exhaust?
Any thoughts on what may be causing these problems?


Transmission shifting issues are routinely misdiagnosed due to engine performance issues, engine doesnt run right the transmission doesn't run/shift right either its really that simple. It looks you might be dealing with a partially clogged cat, that will cause all you describe without throwing a code until it gets so bad it causes a misfire and flashing CEL.
Hook up a mechanical vacuum gauge to the engine, to insure proper readings, if its indicating a bad cat its time to go a bit further down that road.

http://www.aa1car.com/library/converter.htm


Agree on possible bad converter. A good sign is your MAP readings at part throttle that aren't coming back up.
 
I appreciate everyone's responses, except Donald; why would you even post in my thread? This car was widely praised and respected - even winning north american car of the year - precisely because it wasn't the typical [censored] GM had been spewing out.

Sounds like most everyone agrees with my gut that it's an engine problem, not a transmission problem. I actually don't have a mechanical vacuum gauge, so this sounds like a good excuse to go get one. That could at least help us figure out if its the cat vs a sensor.

I'll keep bad ground in the back of my head too. Car is driven on salted roads all winter and, while it's not badly rusted, there is some corrosion in the engine bay area.
 
Originally Posted By: VetteElite
I appreciate everyone's responses, except Donald; why would you even post in my thread? This car was widely praised and respected - even winning north american car of the year - precisely because it wasn't the typical [censored] GM had been spewing out.

Sounds like most everyone agrees with my gut that it's an engine problem, not a transmission problem. I actually don't have a mechanical vacuum gauge, so this sounds like a good excuse to go get one. That could at least help us figure out if its the cat vs a sensor.

I'll keep bad ground in the back of my head too. Car is driven on salted roads all winter and, while it's not badly rusted, there is some corrosion in the engine bay area.


Probably the cheapest diagnostic tool you can get and incredibly useful but often overlooked these days. Lots of videos and articles on how to get the most out of it, you will be amazed at what something so simple can find out for you.
 
Even though the car is not showing a CEL, scan the car anyway.

I don't know about GMs, but I've had VW/Audis notify me of pending issues that where not yet bad enough to warrant a CEL.
 
The talk about the throttle position sensor got me motivated enough to pop the throttle body out to inspect and clean. I had previously been ignoring this since it was last done less than a year ago - I figured it was still fine. It wasn't too bad, but there was some real black, sticky gunk around the butterfly valve pivots and on the area where air goes past the throttle plate at idle. I gave it a really thorough cleaning - you could eat off it now - did a full idle relearn procedure as suggested earlier, and car idles amazing now. Took the car on a little drive to the grocery store and still noticed a little roughness, but it has improved a ton. Could I have had a sticky throttle plate from that blank gunk causing some weird issues with the engine?

I'm still not convinced the problems are fixed. Something about if something seems too good to be true... I'm still going to get a mechanical vacuum gauge to check it out and just to have it for the future. Can never have too many tools! Going to drive it for a few days and see where I'm at with the issues cropping back up or not. Stay tuned...

knerml - I have scanned even with no CEL and still got nothing. This morning before looking at the throttle body I hooked up my scanner and took some graphs of fuel trims, MAP readouts, and MAF airflow readouts. Airflow reads seemed pretty normal - MAP was correctly calculating the outside pressure and MAF seemed to be correctly measuring airflow (as in, nothing seems too out of the ordinary. About 4 g/s at idle which would be reasonable for a 3.5L and increasing as expected when RPMs are elevated). Combined fuel trims ST and LT were never outside of the +/- 10% range.

The suggestion about VVT intrigued me because I don't know much about VVT. My car always seems to have advanced timing. About 18 degrees at idle and increasing when the throttle is pressed. I have no clue what that means. Valve timing and VVT is one of the areas of engines that I have very little knowledge about and little ability to conceptualize in my head.
 
Last edited:
See what happens if its good, cheap and easy enough fix, pat yourself on the back. The ECM reads a MAP sensor, its always best to double check with a real gauge to either verify of dismiss the readings.
If a port was plugged with junk at the throttle body that that sensor uses it may have caused an erroneous MAP reading and the ECM responded to it, a MAP sensor reads absolute press not vacuum like a gauge so the readings will be different, I don't know how the software in the scan gauge is working but it has to be converting volts=psi to Hg vacuum. time will tell if its all well.
 
I wanted to update the thread since I seemed to have solved the problem.

Determined that there was no exhaust restriction (good thing - transverse V6 so I don't want to have to work on the rear bank of the engine), so I thought hard about my symptoms and took it on some deliberate test drives to narrow down exactly what I thought it was doing. Then, doing more research, I zoned into it being some issue with the TPS or Throttle Body. Ordered a new Hitachi throttle body (the actual OEM part) with TPS included, runs like a dream.

I don't know if it was a bad TPS or a problem with the throttle body (either broken teeth on the cheap plastic gears used in the motor or a sticky plate), but the surging and hesitation is gone. Still has some rough shifts, but I'm chalking that up to it being a GM 4 speed. Unfortunately, I haven't gotten to hook up my scanner to look at some data now with the new throttle body, but I will. Took a 200 mile trip for work in the car today and it was a pleasure driving the car with better throttle response and no surging on the highway!
 
Status
Not open for further replies.
Back
Top