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Discussion Starter #1
I have been chasing the cause of this problem for almost 2 months now, and can’t seem to find it. I have a 97’ s10 2.2, the engine has a slight miss and is running rich. The computer is just throwing a p0300 code. I have replaced/tested all the ignition components from the ICM to the plugs. The plugs and wire were replaced with ACDelco. I tested the spark on every wire, easily jumped the gap on my tester. The fuel pressure was tested, that was good, you can hear pump kick in to prime when key is turned on. The fuel filter has been replaced, the injectors were all tested, new o-rings installed. I have replaced all the sensors except for the Camshaft, crankshaft, and the EGR valve. The timing cover was removed and everything checked out in there. Hopefully y’all can point me in the right direction.


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94 4x4,01 Blazer
2001 Blazer 4dr 4wd LT
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vacumn leak
 

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Discussion Starter #3
We did end up finding some vacuum leaks and fixed those, but it sill has the miss. The truck also sometimes has to be cranked a few second and the gas pedal feathered for it to start up.


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Discussion Starter #4
Also I should add, we just redid the head gasket. The head pressure checked, planned, and a compression check done once put back on and it was good.
A hesitation comes along with the miss as well, you will go to take off and the truck will wanna fall on its face, but it’ll come out of it. The miss also only happens at idle, clears up as soon as you give it a little throttle.


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94 4x4,01 Blazer
2001 Blazer 4dr 4wd LT
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Head job- did you put the GROUND wires back on the back of the heads?
 

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Discussion Starter #7
I just replaced both coils on the truck a few months ago, when I tested the spark they are were firing. All the grounds were put back. The truck seems like it is getting to much fuel, it doesn’t get the best mileage and when you pull the plugs out they are wet. When you go to start it, you also have to feather the gas, and it has to crank for a few seconds. The truck will start but it’s rough and has to take a second to get a steady idle.


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Ok, I've seen this before. You probably need new timing gears. They wear down and get slop in them, and causes them to run rough. I have even seen them jump time they get so loose. It happened to me once when I was working on one, but yea, check the timing gears. I don't know why gm made them so soft and the chain so hard, but try that. Hopefully you find the problem.
 

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I just replaced both coils on the truck a few months ago, when I tested the spark they are were firing. All the grounds were put back. The truck seems like it is getting to much fuel, it doesn’t get the best mileage and when you pull the plugs out they are wet. When you go to start it, you also have to feather the gas, and it has to crank for a few seconds. The truck will start but it’s rough and has to take a second to get a steady idle.


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After just ready this I'm telling you check your timing I would be willing to put money on it
 

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Discussion Starter #10
Well funny you say that, I just had my buddy tear it apart last night and check the timing, it was on and the timing set was new. I should tell yah I bout the truck 2 months ago and seems like the person I bought it from was chasing this miss as well


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94 4x4,01 Blazer
2001 Blazer 4dr 4wd LT
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wet plugs and you have good fuel pressure?- you need a scan tool and check your fuel trims- no check engine light? you have an air leak that is causing the ecm to richen the engine up- look/check for exhaust leaks from mid pipe(after cat) forward.
 

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Discussion Starter #12
The guy I took it too has one on the fancy scan tools and said the fuel trims were good and injector pulses were on. The whole exhaust from manifold back was replaced when I bought the truck, last owner cut out the cat and horribly welded in a pipe. When you go to start it you have to press the pedal down a few times and then when it does go it kinda sounds like it’s gonna die, but comes out of it. Then once it’s going it, the misfire starts, and only happens at idle. I was driving it home last night, and I would stop at a stop light and go to take off and it would almost die when I hit the gas, but it sputters through it, only happens from a stop.


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94 4x4,01 Blazer
2001 Blazer 4dr 4wd LT
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'97 2.2 - no cat - do you have the post cat o2 sensor installed?
 

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Discussion Starter #14
Yes, I put went ahead and bought both the O2 sensors and replaced those when I installed the new cat and all new piping from manifold back to the muffler


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Discussion Starter #15
Update: I found another vacuum leak, got the all fixed. I put a vacuum tester and started the truck and only was having around 6in, I think I read somewhere it was supposed to be 17-20in. I don’t know if it matters but while running I unplugged the FPR and the vacuum didn’t change, and the idle was sill bad with the slight miss, does that mean the FPR is stuck closed?


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normal vacuum at idle should be up towards 20". But 6" something is wrong. It's not breathing right. Either constipated or choking. A clogged up exhaust would do that. But you say it is all new. How about a rag sucked into the tube between the air filter and the throttle body?
 

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Discussion Starter #17
Well i know everything from the manifold back is good, haven’t even put 500 miles on it since installed. I haven’t checked to see if anything is trapped in the intake tubing, but I will. The truck will run fine once you get it off of idle, but when idling it has a very slight miss it sounds like. The miss won’t be constant, it could go away for a few seconds and then be right back. I checked for vacuum leaks yesterday with the smoke method and only found one and I fixed it, the brake booster hose had a crack. Could it be a vacuum leak other than somewhere in the engine bay? Weak vacuum pump? Clogged EVAP system?


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Come to think about it more, blocked intake or exhaust would get worse at higher RPM. Head job maybe would effectively lengthen the valve push rods. Maybe keeping the valves from completely seating. That might explain the low vacuum and that it is better warm and off idle. I don't remember if specific to this engine but there are spacers to fix that. Cam lifters stuck? Marvel Mystery Oil in the oil might clear them. Cylinder Leak Down and Wet/Dry Compression Test. would identify if it's valves or rings. Excessive oil pressure?
 

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Discussion Starter #19
When the head job was done, we did a compression test and all cylinders had good compression, can’t quite remember the numbers thou. I didn’t know if there was a secondary air pump that maybe helped along with the vacuum of the engine, maybe it could have a leak or starting to fail? I am sill leaning towards a leaky injector, haven’t had time or resources to perform a leak down test yet. The start sill sometimes takes awhile to start and if you hold it to the floor, it’ll start up and stumble but eventually get to its feet. Also may have something to do with the EVAP system I don’t really know.


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" the injectors were all tested, new o-rings installed". When you apply vacuum to the fuel pressure regulator you should see a reduction in fuel pressure. Simply disconnecting the vacuum line to the fuel pressure regulator will not make any difference The engine has low vacuum as if you are wide open throttle, so the pressure will always be at maximum, explaining the wet plugs and running rich. Do you have a scan tool reading for the MAP sensor and what is it? Put on a fuel pressure gauge. With the engine running when you apply 18" vacuum to the fuel pressure regulator the fuel pressure should go down. Really strong lungs might do it but a vacuum pump is easier. You could use another running engine and hook the FPR to the donor engine vacuum (like a jumper cable, but for vacuum). Another Test:Turn the key on to build pressure, note the pressure. Turn the key off and see how quickly the fuel pressure drops. It should hold pressure. If it drops you have a fuel leak somewhere. Could be the fuel pump check valve. That would explain the extended crank but not the low vacuum. There is a 1/2 inch tube on the underside of the intake plenum that goes to the rear valve cover passenger side. It causes a major vacuum leak, but that would cause high +18 fuel trim (too lean). It is easy to miss because it is hard to see. A cheap scan tool, and a fuel pressure gauge are going to save you money. Without them you are flying blind. You don't need a scan tool that is interactive or has ABS and body computer capabilities to diagnose engine problems, though nice if you have the budget. ZR4 OBD2 Code Reader is $45 at Harbour Freight. I bought a scanner on Amazon several years ago for $79 that has served me well on Toyotas and GM. I think I paid $29 for a fuel pressure tester kit. Less than you have already spent on randomly replacing parts by guess.
Pittsburgh Automotive Cylinder Leak-Down Tester $25 need to hook up to air compressor though.. You could have valves not completely closing because the push rods are too long since the head was planed and still has adequate compression, that might simulate like the timing was off In conclusion, because I see I rambled. You have low vacuum causing high fuel pressure so running rich. Maybe the plenum to valve cover tube (PCV hose) Maybe the fuel pump check valve is causing long crank when cold. You need to see the live sensor readings and fuel pressures.
 
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