hesitation under load

Douglas Walker

Registered User
On my 90 sc i have a hesitation when accelerating from a stop or cruising speed. Im also only getting about 6-7lbs of boost at wot

If im in park and punch it off idle. It have a 2sec delay before the tach moves. You can watch the vac gauge go from -18 to about 1lbs of boost before the car reacts. And this is basically what happens when driving.

Ive pulled the intake off and all the piping resealed it, new vac lines, new plugs, wires, fuel filter. checked fuel pressure, 32 at idle, 50ish under boost. The motor has about 30,000 miles on it and has run like this ever since the old motor blew head gaskets. The motor was put in back when they would warrantee headgasket failure, so the car sits for long periods of time.

Ive changed DIS, Cam Sensor, coil pack, checked for broken balancer, tps voltage is in recommended range.

No codes are showing up. Im at a loss, what else can i check.

Would plugged converters cause an issue, Im sure there was alot of antifreeze pumped through them years ago.

O ya the car is completely stock, current odometer reading is 55,000


Thanks
Doug
 
you could unbolt the exhaust and just see how it drives...

not sure but maybe an IC problem or a towel or something in the tubes...have you checked compression?

Does it build boost in nuetral?
 
Tubes are clear. It builds boost but only about 7lbs doesnt matter park neutral, drive. I thought about unbolting the exhaust but i was hoping i could use a temp gun and check before and after the cat but im unsure if that would show anything since i dont know what the difference should be if any?

Compression is good. The car run and drives fine just lack of power and a hesitation. No bucking or popping.
 
Maybe a fuel flow test would be a good idea...and I would just unbolt the exhaust because its easy to do and will rule out exhaust restriction
 
If cats are plugged, one way to check is to point a temp gun at them and then before/after to see what if any kind of offset may exist temp wise.
 
if the cats are plugged, you should find the down tubes extremely hot, they'll even smell really hot.

Is this an auto or 5-speed car?

Does the bypass valve move smoothly? If it were to hang open it would build boost slowly. Do you feel that the engine RPM's match up with boost pretty well? If so are you sure the transmission isn't slipping? even low on fluid if an auto?

Does the engine sound like it stumbles when you transition to WOT? If so, it could be overly lean, signifying a vacuum leak somewhere.
 
Tubes are clear. It builds boost but only about 7lbs doesnt matter park neutral, drive. I thought about unbolting the exhaust but i was hoping i could use a temp gun and check before and after the cat but im unsure if that would show anything since i dont know what the difference should be if any?

Compression is good. The car run and drives fine just lack of power and a hesitation. No bucking or popping.

Since this all started after a rebuild, I would suspect the cam position scyncro stalk was not installed correctly. If you have a repair manual it explains the correct way to set the stalk/cam sensor vane, or you can search it here.

It would be pretty easy to set the motor to the correct position and remove the sensor to see if the vane is correctly positioned.

David
 
The car runs rather hot, ive been trying different things to bring the temp down. With the stock IC, checking it with a temp gun after normal driving for 30mins, I got readings of 165 degrees at the top of the IC. Which seemed higher than it should be. Its been so long since ive messed with an all stock sc so it may be fine.

Mike
I checked the bypass and it works smoothly with no catches. after the hesitation the car reacts fine just feels like it lacks alittle. Auto tranny, No slip in the tranny. I checked and rechecked for vac leaks. brake clean carb clean, pressurized the system and used soapy water to look for bubbles, etc.

David
The cam position was something I kept going back to. I havent actually checked it but ive been debating. When I run a KOER it come up with a 6cyl, I wasnt sure if it was 180 degrees out if it would still come up with the 6cyl indication.

Im gonna check the exhaust temps and probably unhook it. One question I had, in other vehicles ive own. When the cats get to plugged you usually cant get up to highway speeds becuase of the back pressure. I ran the sc up to 90 over the weekend and it seem to accelerate fine after the hesitation of course.
 
I'm pretty sure if the cam position sensor is off the car wont run...mine was 180 out once and it just choked to death
 
>When the cats get to plugged you usually cant get up to highway speeds becuase of the back pressure. I ran the sc up to 90 over the weekend and it seem to accelerate fine after the hesitation of course.

Sure, on a N/A car :)

You said it still builds 6~7 psi boost...maybe there is a partial restriction the blower can overcome to a degree - see what the temps are after a run.

Or take the belt off and see it can even get out of it's way...
 
Any reason to not trust the stock boost gauge? I would expect at least 10 -12 minimum on a stock setup.

Any less then that is going to indicate a vac leak or belt slip. That's always possible that on initial snap of the throttle, the belt slips.

I would expect a lean condition to impact the blower psi, but if you are running lean (killed your o2 sensors with coolant) you could see some on throttle hesitation.

If you're not sure on the cam sensor you can always unplug it and see if the car runs better with it disconnected.

Under hood temps are regularly high enough to burn even with the coolest running motor. The hood traps everything in.
 
Ive put all new belts on it and the tensioners are still tight.

when the car is off the boost gauge reads 1" of vac. So it may be a little off. Ill hook a known gauge up to see if its comparable.

Ill pull the O2's and see what kind of build up is on them, You would think they would throw a code sooner or later

Ill try disconnecting the cam sensor and see if it helps

Even though the fuel pressure checked ok out, Im still questioning the fuel pump.
 
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I finally got around to temping the exhaust

Engine temp 220
driverside at o2 sensor 515
pass side at o2 sensor 515
after cat driverside 415
after cat passenger side 424
b4 res driver 284 pass side 300

anyone know what the difference should be before and after the cat?

I did talk to the owner and it wasnt a head gasket issue, it was im guessing detonation because they blew a hole in the piston and drove the car for another 10miles or so. With oil dripping out the exhaust. So what would that do to the cats and or 02 sensors?
Doug
 
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my biggest issues have been harmonic balancer fuel pump and fuel pressure regulator , dirty mass air flow sensor can cause hesitation but low boost sounds like spun harmonic balancer , mine hb spun so far i had no boost at all
 
Looks like exhaust is flowing. next stop, fuel. If you know your Cam sensor is properly set (you disconnected it and the car behaved the same) then you next need to consider fuel flow. You can get a fuel pressure gauge, sometimes rent them, that will screw onto the nipple on the fuel rail. Then tape the gauge head to the windshield. Drive around and watch the pressure.
 
exhaust temps.

I always thought that the exhaust gas temperature should be higher after the cats. They are helping to burn or convert combustion byproducts. If the temps are higher before the cat than that should indicate a plugged cat. Maybe I'm off base here but this is what I was taught. I would drop the exhaust at the manifolds and drive it. The exhaust is heavy but with two jacks its doable with one person.
 
I guess I can't say for sure. At those low temps, I'm not sure the Catalyst has lit off yet or not, so it may not be generating any heat. But yes, digging around google a bit it does seem that normally operating converters should cause a higher output temp than their input temp.

Oil and Coolant can both damage a converter. Oil can plug it. So I revise my prior statement. I would test the cats one more time by driving around to get good heat into them. Then come back and use the infrared thermometer. Take before, after and on the cat housing itself.
 
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