1994 Chevy Blazer Jerking when driving.

Tiny
BLAZERGUY727
  • MEMBER
  • 1994 CHEVROLET BLAZER
Transmission problem
1994 Chevy Blazer V8 Four Wheel Drive Automatic

Ok, while I am driving, my truck will all of a sudden jerk suddenly. Almost like it is slamming. It really only happens while I am in overdrive. When I press the gas to faster it will jerk/slam. So I have been driving in regular drive. I talked to mechanic and he said it is most likely my torque converter. Any Ideas?
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Wednesday, March 4th, 2009 AT 5:52 PM

26 Replies

Tiny
JAMES W.
  • MEMBER
When the jerking condition exists do the engine rpms jump for an instant or does it seem like you've just hit the brake once? It could be the convertor or the convertor lockup solenoid in the transmission. Another thought is the trans may be dropping out of OD into drive and back up in an instant. Please advise.
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Wednesday, March 4th, 2009 AT 8:03 PM
Tiny
BLAZERGUY727
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Well I'm not sure. My RPM gauge is always jumping. Its almost like it drops out of OD for a second then right back in. Its enough to almost throw me into the steering wheel. Its a heavy duty jerk. I just replaced the fuel filter because I thought maybe it was a stall type jerk but that didn't do anything. I narrowed it down to either a electrical problem or a transmission type problem.
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Wednesday, March 4th, 2009 AT 9:01 PM
Tiny
JAMES W.
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Your tach gets it's signal from the ECM. If the ECM thinks the engine rpm has dropped, it may be telling the trans to downshift even for just an instant. It may be such a thing that it is going into drive before it's left OD. Just a theory. If you have access to a mobile diagnostic unit like an OMB 2 or similar, you can monitor rpms and shifting patterns going down the road. Let me know your thoughts on this. I really don't it's a torque convertor problem unless I'm missing something here.
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Wednesday, March 4th, 2009 AT 11:42 PM
Tiny
BLAZERGUY727
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Well this morning I payed for attention to the jerking. It is almost like my motor is misfiring or something along those lines. When I press the gas to get up speed my motor sounds like its sputtering. I'm not quite sure how to explain it. Im begining to think it has nothing to do with my transmission. I just replaced the altenator. No luck. Could it be a spark plug issue? Maybe to much fuel. I replaced the fuel fiilter yesterday. No luck. It really only happens bad in overdrive. When I'm in regular drive it does it kinda like its being pokey. Its kinda like. ERRR JERK JERK ERRRRRRR JERK EEERRRRRRRRRRRR JERK. If that makes any sense. Make no mistake when I mean it jerks, it jerks. Kind of like if you slam on the brake really fast and let off.
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Thursday, March 5th, 2009 AT 8:54 AM
Tiny
JAMES W.
  • MEMBER
OK, now it's making sense. If I'm correct this condition is happening when you are in drive also, BUT, when you're in OD, the engine rpms and power curve are lower so the hesitation is more pronounced. I'm thinking now that you have a "dead spot" in your throttle position sensor (TPS). Apparently, you are not getting a check engine light. Cruising down the road the throttle is usually held pretty much the same position. After a while this will wear a dead spot in the TPS. Once in a while when hitting this spot, the signal to the ECM drops out for just an instant and shuts down the fuel injection. All the ECM knows is that you took your foot off the gas, even though you didn't. We can test this theory with an ohm meter at the TPS. Testing accross TPS terminals A & B for a V8 or B & C for a 2.5. The meter reading should rise and fall smoothly from idle to full throttle. If it falters at all, there would be your problem, or you could just change the sensor. They're not that expensive. Also, this condition could be happening so fast that the ECM can't pick it up to register a trouble code. Let me know what you find.
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Thursday, March 5th, 2009 AT 3:42 PM
Tiny
BLAZERGUY727
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Wow you hit the nail right on the head. Would you know where the TPS would be on my truck? I've never heard of it before. What does it actually do? And would it be something I could do by myself or would that be a shop job?
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Thursday, March 5th, 2009 AT 4:01 PM
Tiny
JAMES W.
  • MEMBER
You should be able to change the TPS yourself with basic tools. The TPS is mounted on the passenger side of the throttle body unit. It's on the opposite end of the pivot shaft that the throttle cable mounts to. Usually held on by two TORX head screws. It may or may not be adjustable, but if it's adjustable, you can usually set the idle speed by ear. If you have, or have access to, a fax #, I can ship you a location sheet. I can attach it to this sight, but they don't come through very well.
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Thursday, March 5th, 2009 AT 7:14 PM
Tiny
BLAZERGUY727
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That would be great. 517-647-2670, address it to Willy if you would. Thank you. Should the TPS make my truck not so pokey. I also have anothe question, should my fan (the one infront of the motor) be able to wiggle? I went to an autozone and he gave me alist of problems. Not sure if hes accurate or not. Said my water pump was going bad because of moisture on the bottom of it, my clutch fan was bad, my intake manifold gasket was bad, and my belt is bad (i know that bad.)
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Friday, March 6th, 2009 AT 9:05 AM
Tiny
JAMES W.
  • MEMBER
Willy, fax is on it's way.
On your water pump. Located on the bottom of the pump is what's called a "weep hole". You usually need a mirror to see it. This hole allows coolant to escape from the pump before it gets to the pump bearing in the event the pump seal starts to leak. If the pump seal is indeed leaking, water coming out the weep hole, it means the pump is on it's last leg and should be changed. The "wiggle", could be the pump bearing, the fan clutch bushing, or a combination of both. To answer your question, neither should wiggle.
What is supposed to be wrong with the intake gasket? Is it supposed to be leaking coolant or air? GM V6's are noted for leaking, but not V8's.
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Friday, March 6th, 2009 AT 12:18 PM
Tiny
BLAZERGUY727
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I got your fax. So I probably replace the water pump anyway. I am not sure what is wrong with my intake gasket, the guy just told me it was bad. I think he might of heard air or something because I have two intake bolts missing (ir. Broke off). I have to take that to a machine shop because half of the bolt is still in the manifold. So my plan is now to replace the water pump and the fan clutch. I can wiggle the fan clutch a 1/2" in both directions so I will be replacing that. Today.
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Friday, March 6th, 2009 AT 12:33 PM
Tiny
JAMES W.
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Good, keep me posted.
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Friday, March 6th, 2009 AT 12:50 PM
Tiny
BLAZERGUY727
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Will do.
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Friday, March 6th, 2009 AT 1:00 PM
Tiny
BLAZERGUY727
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Ok, I replaced the TPS with no luck. It is still jerking/chugging. Could it be a spark plug issue or fuel pump? I guess I am at a loss here. I am now getting a loud ticking noise but I think thats my water pump or serpentine belt. My belt has a chunck missing on one side. I'm having more problems than ever. It just seems to be really boggy, and it is still chugging/jerking. Any ideas?
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Saturday, March 7th, 2009 AT 10:44 AM
Tiny
JAMES W.
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I have so many post active right now, I have to re-read them to make sure what I'm dealing with. In going over your's, I had a thought on the intake manifold leak. The ECM will compensate for a vacuum leak by richening the fuel mixture trying to maintain the 14.1 to 1 ratio. BUT, the ECM's ability to richen is only up to a certain point. If cruising down the road and the ECM has richened to it's maximun ability, and you accelerate just slightly (open the throttle plate), the mixture will lean out too much and it will "lean cough" or "buck". You mentioned you had some broken bolts on the intake. Are these the bolts that hold the manifold to the head? If I'm correct, this would explain the problem. You can test for intake leaks as follows. Some people use a spray bottle of water for the test. I've always prefered a spray can of ordinary carb cleaner, but you have to be careful, we don't want a fire. With the engine at a warm idle, liberally spray down the area where the intake manifold meets the cylinder heads and around the throttle body. If you hit an intake leak the engine rpms will jump up for a few seconds, then return to normal. I wish I had thought of this earlier, but I didn't pay enough attention to the intake issue. Let me know what you find.
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Saturday, March 7th, 2009 AT 12:39 PM
Tiny
BLAZERGUY727
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I have made a mistake. The bolts are missing on my EXHAUST manifold. Not my Intake manifold. My thought was, my oxygen sensor in the original. Would that cause my truck to run rich? I'm a firefighter and I run my truck alittle faster than most. I need my truck running top shape. Lately I havent been able to run my regular speeds. This bucking problem is becoming most annoying haha.
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Saturday, March 7th, 2009 AT 2:12 PM
Tiny
JAMES W.
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Firefighter, huh? Not to patrinize you, but you guys are my hero's. I was in a volunteer dept. Many years ago. It's a lot of training.
Exhaust manifold, you say. Well that blows that theory. Yes, an O2 sensor can make an engine run rich or lean which can screw up the fuel mixture. They can do this and still stay enough in range where they won't set a trouble code. It's rare, but it does happen. I'm beginning to feel like an "ene', mene', mine', mo', parts replacer here. I wish I could run the truck down the road just once. If you had access to a mobile diagnostic unit, we could plug it in and take a short road trip. This will allow you monitor the sensor perameters under realtime conditions. An O2 sensor is like a battery. It puts out.1 volts at cold idle(rich) to.9 volts hot (lean). Is this kind of a test possible? Note: The O2 sensor can also be tested when removed and heated with a propane torch on a volt meter.
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Saturday, March 7th, 2009 AT 3:22 PM
Tiny
BLAZERGUY727
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Well thank you. Been a firemen for about 4 years now. The 727 number in my name is actually my fire number. Lyons/Muir Firefighter 727. I dont know of a way to do that test around here. We have a local autozone but they only do battery and alternator testing. Well I was going to replace the O2 sensor anyway. Where would it be located on my truck? Haha yeah, ive been replacing parts for a few days now. Im getting use to it haha. I guess the next thing will be an O2 sensor haha
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Saturday, March 7th, 2009 AT 3:43 PM
Tiny
JAMES W.
  • MEMBER
It's a good thing you brought Autozone up. They will do computer diagnostic checks for free. They may even let you barrow their unit. This would sure help narrow this problem down. Quessing makes things difficult not to mention expensive for you. Let me know.
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Saturday, March 7th, 2009 AT 5:06 PM
Tiny
BLAZERGUY727
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What diagnostic test should I ask for?
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Saturday, March 7th, 2009 AT 5:07 PM
Tiny
JAMES W.
  • MEMBER
Looking for any stored trouble codes and the voltage output of the O2 sensor(s).
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Saturday, March 7th, 2009 AT 5:37 PM

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