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Boost pressure drops.

FarmerJoe

FarmerJoe

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Finland
I have a 1995 Suburban, 6.5 turbo diesel and it drops the boost pressure. When I accelerate, boost rises up to 3-5 psi and it drops down to zero. I get off the throttle and accelerate again, boost rises.
If I ride "nicely", boost will stay in 3-4 psi on highway. Any suggestions ?
 
Does it drop off while still accelerating? There needs to be a load on the engine to maintain a steady boost.
 
Yes, it drops while accelerating. Then off the throttle, accelerating again and while accelerating, boost rises max. 5 psi and drops.
 
Make sure the clamps on the rubber tube between the turbo and intake are tight. 3-4 psi cruising is normal for a stock system.
 
What have you done to it? Any mods? Stock control? Check the vacuum pump to see if it pulls 19-20" of vacuum at idle, check exhaust for restrictions, you should get more than 5 psi at WOT.
 
Does the service engine soon light come on? Could be a bad wastegate solenoid. If the ECM detects a problem with the turbo side, it will default to no boost.
 
Sounds normal to me. The engine needs to be under load for turbo to spool and raise PSI. If your boost is droping while accelerating medium to hard then you got a problem.
 
Car is a stock, except the exhaust. I have checked all the connections, manifold, turbo, vacuum pump etc. Nothing wrong there. The problem began after one of the batteries went short circuit. I tested the other battery and replaced alternator. After that, boost pressure startet to drop in heavy acceleration. I thought it was the air cleaner, I took of the Accel cleaner and put the original box. It helped a lot, never wouldn´t believe that OEM beats high perf. cleaners, but it does.

It did lid the engine light couple of times before the battery went dead, it happened when I rode hard for little longer, highway speed around 140 kmh. But I never noticed any problems with boost. Before the battery incident, on acceleration boost went up to 8 or 9 psi max and dropped from there around 6 psi and stayed there as long as I keep the pedal on the floor.

I have now a new MAP sensor and pressure solenoid, I`m going to change them today and check the vacuum pump.

Is next thing the ECM ?
 
Burning oil said:
Sounds normal to me. The engine needs to be under load for turbo to spool and raise PSI. If your boost is droping while accelerating medium to hard then you got a problem.
Click to expand...

Yes, boost drops when I´m accelerating. If I drive like 80 kmh and I want to pass a car, I accelerate, transmission kick down works fine, revs go up, boost goes up, but it drops almost immediately. Even when there is a load on the engine when I´m passing other vehicle. It has never ever done this before. I had a problem with the pressure solenoid few years back, then it didn´t rise any boost. I replaced the solenoid and everything worked great. until now.
 
You could have tweaked the ECM but check the pump first, then maybe replace the wastegate solenoid. Definately find out what codes are being stored.
 
I have replaced the solenoid, no changes. Checking the codes now. There seems to be two pressure sensors, one above the manifold with black connector and second in firewall with green connector. How these affect to the boost. The one in firewall seems to be connected to the boost solenoid with vacuum hose.
 
Post a picture of your engine and the hose routing. Sounds like you have some hoses hooked up wrong or somehow you have an S engine.
 
There is a sensoir on top of the manifold. Top of the sensor is GM logo and numbers 466, above the white connector, the is numbers 866. 7350
Sensor_manifold.jpg

On the firewall the is two sensors, on the right is pressure sensor for Autometer boost gauge. Left with the green connector is connected to the boost solenoid ( blue nylon hose )

Two_sensors.jpgBoost_solenoid.jpg
 
I have had this car past eleven years, I have not make any modifications to it except, added gauges to the a pillar, 4" exhaust.

Boost solenoid and sensor on the firewall share the same blue nylon hose which is coming from the round rusty thing from the manifold top.

Engine.jpg

I haven`t had any problems with boost before, except once, I had to change boost solenoid. Problems began when one of the batteries short circuited.
 
For starters that isn't a 95 engine, and yes it's an S engine. It is at least a late 96 or newer engine fro mthe A/C compressor on the left side and the dual t-stat housing. This engine is in a SUBURBAN?
 
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