Engine runs great...then bogs under boost.

torino1973

Registered User
Hello everyone,
Hoping to get an idea here. recently replaced a broken balancer, did a tune up and put headers on my 94SC. The car ran perfect prior to the balancer braking. It's back together and runs perfect at idle and when driven without boost. As soon as you accelerate hard enough to hit boost the engine bogs...kind of like when a cat is blocked (but that is not the case). The car does not die it will just run in that boggy mode until you let up on the pedal and then it runs perfect again as long as you stay light on the pedal. I took off an IC tube and the blower blew strong for the 2 secs it was running. I have not found any vacuum leaks...running at 19-20 both on the car gauge and on my diagnostic meter. The timing is right where it should be. The engine light temporarily comes on after a bog episode but never stays on long enough to do a test. My next step will be to check the fuel pressure. Can anyone steer me in the right direction??? I'm really trying to get this car back on the road for @ least a little while before winter.
THANKS!!
 
Have you checked fault codes?

I have same problem, O2-sensor was and fuelpump relay were broken. I just need new relay to my car. My local part store didnt have it so it takes about two weeks to get car running.
 
If it was just bogging under boost, I'd say it spark plugs.
But then I read more details of your problem and not sure any more.
 
i had a similar problem earlier this summer with my 91 bogging under boost, but ran fine if you eased into the throttle.turned out my fuel pump could no longer keep up with the motors demands.check the fuel pressure as you said while driving under boost.mine dropped off badly while under boost instead of adding psi.
 
90% of my experience with the same issue has always come when I have a spark plug boot come loose, or plugs going/gone bad.

I had similar problems as you mention and it was due to 1 spark plug wire on the fringe of falling off.

Check the easy stuff first.

-Tim
 
Cam position sensor

My '95 drove me nuts doing that about a year ago. Pull your codes. I don't remember the code number, but if it's "misfire on cylinder XX", that's it. The engine goes into default mode where it's guessing the cam position to fire the spark. When you rev it up, your ignition timing isn't advancing because it can't determine the cam position, hence the bog and the CEL. CEL will go back off after a while after it settles in. It's a $30 part and only takes about 5 minutes to change.
I was also getting an O2 rich code and a code on the Fuel pump secondary citcuit (PCM). All of it stopped when I replaced the Cam sensor. It throws the ignition timing off and pretty much pi$$e$ the whole ECU off.

Daspanka
 
Idles But breaks up under load.

Thanks for all the input,
Daspanka, I was finally able to pull a code and got "secodary fuel circuit " code and nothing else. I have already changed the Cam Sensor. As for the fuel pressure the car holds at about 40psi while sitting still and applying load(pressing the gas while holding the brake to the point of the car "power braking"). I don't know how the fuel volume holds up under boost. BTW I've already installed a new fuel pressure regulator.
The wires are all tight, but I know what Tim's talkin about ...I had expierenced that in the past.
So I still haven't figured this out.
DOES ANYONE KNOW WHAT THE FUEL PRESSURE SHOULD BE THROUGHOUT THE BOOST PROCESS ?? MY CAR STAYS AT 40 EVEN WHEN YOU ARE BOOSTING.
 
Fuel pump

The primary fuel circuit is the ECU to PCM. The secondary circuit is the PCM to fuel pump. I can't remember what the ratio actually is, but you should be gaining fuel pressure at the rail for each pound of boost (it may be pound for pound). If your regulator is new, and is actuating properly by vacum/boost, then the fuel pump may be going T.U.
There's a connection in the trunk, I beleive in the drivers side behind the cover. Disconnect it and spray some connection cleaner/protector it. Another trick to test is to put a double dose of Lucas Upper Cylinder lube in the fuel. One or two 6oz bottles per 10 gallons. It will not damage O2's or plugs. The Lucas is very oily. If the impeller in the pump is worn and binding, it will lubricate and "free" it up a bit. If the electric motor or shaft is worn, this won't do jack, of course. If the impeller is binding, it could be overloading/overcurrent the motor, causing the secondary circuit failure code. Cheap way to test and it may buy some time before the pump fails completely. But, if it does work, I'd be picking out a new fuel pump.

Daspanka

P.S. I'd put two bottles per 10 gallons to test. It burns completely and won't fowl anything. Give it a few miles. If it helps, you can dilute it down and run a leaner mix of it until you can replace the pump. Hope this helps. Make sure you have a brand new fuel filter on too.
 
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Thanks for all the input,
Daspanka, I was finally able to pull a code and got "secodary fuel circuit " code and nothing else. I have already changed the Cam Sensor. As for the fuel pressure the car holds at about 40psi while sitting still and applying load(pressing the gas while holding the brake to the point of the car "power braking"). I don't know how the fuel volume holds up under boost. BTW I've already installed a new fuel pressure regulator.
The wires are all tight, but I know what Tim's talkin about ...I had expierenced that in the past.
So I still haven't figured this out.
DOES ANYONE KNOW WHAT THE FUEL PRESSURE SHOULD BE THROUGHOUT THE BOOST PROCESS ?? MY CAR STAYS AT 40 EVEN WHEN YOU ARE BOOSTING.

Base pressure should be 38-40 psi at 0 on your boost gauge...it should increase pressure with boost...example 10 pounds of boost you should be getting 50 psi fuel pressure. If your car is holding 40 psi at all times that is part of the problem. Sounds like the vacuum/boost refrence hose isn't attached to the regulator.

David
 
Recheck that you have all the sensors plugged in again. The EEC does have a fail safe mode that causes the car to run real rich and backs the timing way off if one of the major sensors have dropped off. It will feel exactly like your describing.

Out of curiosity, did you unplug the battery when you where doing the work? If not unplug it for 15 minutes or so, then try it.
 
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