Random surge-acceleration Rendezvous 3400

richardnewton

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Shop replaced motor recently with another 3400 on 2003 AWD Rendezvous. New engine ran decently but had poor gas mileage and sometimes RPMs would surge to 3000+. No codes. Surging happened say a few times a week typically on the highway or when parking (e.g. shifting from reverse to park).

Mechanic tried to resolve surging a few times with no luck; not sure what he tried.

Based on internet searches for this issue on 3400 engine, there are a lot of potential causes. Popular issues include: vacuum leaks, IAC valve, TPS, loose electrical connections to IAC/TPS, low battery voltage (loose-dirty battery connections, bad battery, etc.), bad thermostat-air in coolant, bad computer/connectors, fuel pump/regulator/filter, etc.

I tried a few DIY fixes over a few weeks in approximate order (some of this work was general maintenance):

- Cleaned dirty IAC valve (delicate part so use extreme caution). Don't forget good o-ring placement.

- Replaced dirty airfilter & cleaned airbox

- Cleaned MAF sensor (use only MAF cleaner!)

- Cleaned dirty throttle body (both sides of butterfly and circumference-did not remove from car) (use only throttle body cleaner!)

- Cleaned and retightened battery connections

- Searched for and repaired minor vacuum leaks
--- airfilter airbox missing a clip (fixed with zip tie)
--- replaced $9 PCV tube which goes from air intake hose to back valve cover; back connection was trashed and this is weak friction fit (part number on factory sticker of my tube was 24508188 but make sure to look at the picture as there is some confusion)

- Checked throttle cable for smooth operation

- Disconnected cruise control cable from throttle body

- Tightened & cleaned electrical contacts at IAC valve & TPS (all looked very clean)

- Attempted to clean EGR valve and tube (could not remove lol)

- Replaced $29 TPS (delicate part so use caution)

Fix: Replacement TPS appears to have fixed the problem. 500 miles with no surging.

Bonus benefits: Engine runs much quieter, smoother and significantly better gas mileage. Runs fantastic for regular daily driving. Under heavy load acceleration, engine has a bit of pre-detonation pinging; I hope that was from ultra-hot weather and engine re-mapping but won't be able to test for a while.

To do: Some high-mileage 3400 & 3800 engines run much better after cleaning EGR valve and tube. The EGR valve is tough to remove on the Rendezvous. There are two 10mm(?) bolts. When standing at the front bumper facing the engine, the right hand bolt is relatively easy to reach the left hand one is very tough to reach. But, the bolts are pretty easy to reach with easy trick removing coilpack:

 

richardnewton

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About 25 MPG highway and 23 city over past 800 miles. Virtually all pre-detonation has disappeared, probably as computer has reset and remapped. This is AWD model (but AWD and AC are disabled).

Ideas may be applicable for Aztek, Montana, Venture, Silhouette and other 3.4L or 3.5L GM vehicles of the era.
 

Zeroboostbuick

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Right on mate !

Good job and thank you for reporting back. This thread will eventually help someone else with the same issue.

When your engine was Knocking... how many degrees of Knock Retard did you see ??? In the Hot summer days with moderate to hard acceleration, up to 5* of KR is acceptable and if the ECM does it's job the engine will never get hurt. Only loose a little power.

Anything closer to 10* of KR is too much and should be looked into or avoid by not loading the engine very hard.
 

Salvaing

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Richardnewton

I've got exactly this same issue and I tried all useless DIY fixes you describe for 6 months.

Finally (i dont remember where I found the solution) but I bought a new PCM on flashmasters sur eBay.
Very easy to swap and reprogrammed (Security password relearn procedure has to be performed before the vehicle will start or crank).
Instructions are in the box for that.
The new reprogrammed PCM fixed the *%#$ engine issues (surging , hard starts, high idle, weird trans management ...).
You need to provide : VIN + RPO CODES + service number (sticker on the old PCM).

Hope it helps...
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richardnewton

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Right on mate !

Good job and thank you for reporting back.
Thanks!

When your engine was Knocking... how many degrees of Knock Retard did you see ??? In the Hot summer days with moderate to hard acceleration, up to 5* of KR is acceptable and if the ECM does it's job the engine will never get hurt. Only loose a little power.

Anything closer to 10* of KR is too much and should be looked into or avoid by not loading the engine very hard.
Thanks for the tips. I need to get my brothers scan tool this weekend and check. For sure knocking & transmission shifting improved after a few hundred miles of computer learning/mapping.

I thought knocking might be an issue with knock sensor.

Also, cleaning the EGR valve improves drivability & gas mileage on these engines so much that I thought that might help. Given how dirty the other sensors were, I assume the EGR valve could use a good cleaning. I got the "impossible" left-side bolt removed in a couple of minutes using the video above but the right-side bolt head just rounded (I had a good socket completely on but the bolt was very rusty and soft from heat). I will get one of those Irwin bolt extractors and try again...

What else might typically cause knocking when accelerating under load in the Rendezvous?

Richardnewton

I've got exactly this same issue and I tried all useless DIY fixes you describe for 6 months.
I feel your pain

A lot of the DIY fixes I did are unlikely to fix the engine surging issue but are cheap/easy and help performance/gas mileage a lot so I just did them for fun whilst under the hood. For my situation, I decided to prioritize as follows (someone else's situation might be different):

- Vacuum leaks

- Clean and tighten battery contacts & check grounds to frame (low voltage may cause TPS to improperly calibrate at start-up). Make sure battery is good.

- IAC valve (very gentle cleaning might be sufficient and you can test IAC)

- TPS (can be tested but not practical repair for home-gamer. New part is $30)

- IAC valve & TPS connections are clean and tight. Check voltages at connectors

- Check throttle cables are smooth and not biding

- Thermostat (can do some testing on this. Replacement on Rendezvous is painful)

- Clean, tighten, test PCM connections. Replace PCM (programmed to your VIN)

Finally (i dont remember where I found the solution) but I bought a new PCM on flashmasters sur eBay.
Very easy to swap and reprogrammed (Security password relearn procedure has to be performed before the vehicle will start or crank).
Instructions are in the box for that.
The new reprogrammed PCM fixed the *%#$ engine issues (surging , hard starts, high idle, weird trans management ...).
You need to provide : VIN + RPO CODES + service number (sticker on the old PCM).
Hope it helps...
I read that PCM fix helped several people and saw a few recommendations for flashmasters. I was worried PCM was the issue but less worried after figuring how easy and cheap the DIY fix was (about $100 I think). We didn't have hard starts, high idle, or transmission management issues, just surging.

Glad it worked for you.
 

richardnewton

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I just found my ScanGauge II and went for a quick drive.

> So with engine off, engine idling, and during light acceleration KR shows 0 degrees.

> With moderate increase in engine load (moderate acceleration or a moderate hill), I can hear predetonation; the KR sensor starts registering at "precisely" the same time. Moderate acceleration resulted in KR 20-40. Engine felt a bit bogged down.

> Harder acceleration up hills (not pushing too hard) KR got up to 50-70 degrees. C

So it seems the knock sensor is working. And there is a predetonation issue. No engine codes, no pending codes, and ScanGauge reads "ready."

Any suggestions?

This is the Xgauge code on VPW protocol for cars upto about 2003 (newer cars will use CANSF):
Gauge Knock Retard
-TXD 6C10F12211A6(01)
-RXF 0462851106A6
-RXD 3008
-MTH 002D00320000
-Name KR
-Notes Degrees
 

Zeroboostbuick

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When you say 20-40* /// 50 - 70* degrees of Knock Retard, you mean 2.0 - 4.0 ??

Cause anything over 15 degrees of KR is extremely excessive.

5 to 7 is still ok and engine is not gonna get hurt.
 

richardnewton

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Thanks @Zeroboostbuick

The ScanGauge site says the code is in degrees:

» GM SPECIFIC VPW – Knock Retard

So the engine was reading 0 degrees to about 70 degrees during testing. There is a lot of pinging and not much power up the hills. Engine runs great when not under load/under low load.

Is that range not possible?
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richardnewton

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That makes sense. The timing changed seemed to be capped whilst driving.

I will try getting that bolt off the EGR and cleaning the valve/channels tomorrow then.
 

richardnewton

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When you say 20-40* /// 50 - 70* degrees of Knock Retard, you mean 2.0 - 4.0 ??

Cause anything over 15 degrees of KR is extremely excessive.

5 to 7 is still ok and engine is not gonna get hurt.
Update. Vacuum leaks were the final cause of Knock IMO.

The car was leaking some oil. Mechanic replaced valve cover & intake manifold gaskets. And bad water pump.

Car was more noisy, and pinged on acceleration, and had worse drivability. So I checked and saw the suspect air filter box was not properly closed (it is difficult to make sure all the bottom tabs are aligned and top two clips are evenly closed. So I removed the intake hoses and properly closed up the air filter box.

In my quick very cold day test accelerating up and down the neighborhood hills, Vous drove much better and quieter. I hear no pinging accelerating up hills. MAF readings looked reasonable and KR = 0 whole ride.

In the future I still would like to clean the EGR & pipe as that has helped my other 6 cylinder GM engines.
 

richardnewton

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This weekend, I tried once again to remove the EGR for cleaning the valve, pintel & passages. Partially successful effort this time. This service has made several of our high-mileage GM cars of this era run much smoother & quieter.

I think GM used several EGR valves on these engines over the years. And the engine on this Vous was replaced. So your EGR valve may be different.

It was very difficult to reach the two EGR bolts. They were heavily rusted on so massively difficult to break loose. Then with ratchet, could only turn about 1/12 of a turn so removal took forever. Mine bolts were 10mm.

Passenger's side bolt requires a deep socket as the bolt is long. A short socket will just strip the top of the bolt. I also used a very long extension with my hand very close to the firewall. Driver's side bolt is short and required a very shallow socket and miniature driver wrench (say 3" long). Maybe those ratcheting wrenches or articulated drivers would be easier.

Surprise . . . there was a third bold at flexible exhaust pipe! Could not remove that bolt even with massive force and penetrant so was worried about breaking the flexible pipe, which I think is NA. So I cranked engine for a few seconds with EGR disconnected to try to blow out some carbon. Then ran some TB cleaner & brush into the EGR valve and intake duct (I had cleaned the passage with a snake brush and TB cleaner from the throttle body last summer).

Getting the two bolts back in took forever and the gasket disappeared so needed to buy a new $7 gasket. The trick was to tighten the passenger's side bolt most of the way, then the shorter driver's side bolt could catch.
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