98 headache 4.3

I have a 98 4.3 blazer I've replaced about everything mfi,map,tps,maf,plugs wires twice cap rotor ac delco plugs new heads, all gaskets, iat ,coolant temp My problem is like the other guy I read about starts up great driving when engine cold misses {bronco Bull} only low rpms more than half throttle runs smooth after it warms up runs great trouble shooter says misfire no shit! I appreciate anybodys help .

TomB985

Have you checked your fuel pressure? Sounds like a fuel pump to me. I'd suggest buying or renting a fuel pressure gauge and seeing what you're getting for pressure at idle and with the key on and engine off...should be above 55 PSI....

MT-2500

Check cold start fuel pressure.
Check engine running fuel pressure.
Check engine running on the road fuel pressure.
Then shut it off and watch for fast leak down.
Pinch off at rubber part or block off return line and check full pressure.
Later V6 and V8 gm engines should have 60-66 fuel pressure.
Cold start should be 64-65 lbs of pressure.
If you do not have full fuel pressure on a cold start the injectors will not squirt fuel.
Do not leave home without it.
Engine running should be 60-66 lbs pressure depending on engine load.
Full pressure with return pinched off should be 75-85 or more.
And if it has a fast leak down after shut off you have a leak.
Post back fuel pressure readings.

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Remember proper testing gives us the answer to many problems.
MT

MT-2500

Many things can cause a missfire
Low fuel presure.
Lean running engine.
Maf sensor.
Bad plugs and wires and camshaft retart setting off.

Can you feel a engine miss?
Does the check engine light flash?
What cylinders are missfiring?
Any codes?

__________________
Remember proper testing gives us the answer to many problems.
MT

98 headache 4.3

the miss occurs when engine is cold after a few miles and engine has been warmed up runs pretty good. barely feel the miss.only code comes up is multible missfire.the fuel pump has been repalced don't know how long ago previous owner had it done. fuel pump sounds stong and no problem starting hot or cold and idles great never stalls.to check fuel pressure i can use a tire gauge on the relief?

MT-2500

You will need a real fuel pressure gauge.
Borrow or rent or buy one.
Some parts places will rent or loan or sell you one.
MT

__________________
Remember proper testing gives us the answer to many problems.
MT

need_a_new_truck

Have the exact same year and model as yours. Also had the same problem. You have to change the Cam Position Sensor. It is located inside of the distributor. I just went to the junk yard and got a whole distributor with the cam sensor and put it in mine, but I'm pretty sure the problem was just the Cam Position Sensor.

TomB985

while the CPS could very well cause the problem, just throwing a part at it without any diagnostics could simply waste money and piss you off...

I'd follow MT-2500s suggestions with the fuel pressure testing before you drop any money on new parts.

need_a_new_truck

My solution to his problem is not a cure all for all miss fires. Heck, there are a lot of different solutions to miss fires. But if you have a 98 with high mileage, starts fine, idles fine, misfires a lot when the engine is cold and the misfire decreases as the engine gets to operating temp, and has an engine code of multiple misfire....... then I would say it's the cam position sensor. Been there done that.

In addition.....I took my vehicle to a GM tech and had all of the diagnostics tests done on it. $80 later and all they told me I had an evaporative canister problem.....which I knew already.

I also don't want people to think that you shouldn't take your vehicle into a service tech to get it tested because they usually do find the problem.....but I'm just saying that they did not catch this particular problem on their diagnostic machine.

98 headache 4.3

my missfire problem has been solved so far. ended up being clogged cats replaced and is running smooth plenty of power no more bucking and jumping . thanks to everyone for there help and ideas I've learned a lot from this forum

kcrowe

Just flunked emissions test on my 96 Blazer due to same code reading. How did you diagnose that it was clogged cats?

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K.A. Crowe ;^)

maxwedge

A back pressure test done thru the o'2 sensor port is the most common method, another is to check the inlet vs outlet temp of the cat, there should be 100 degrees difference once the engine is at operating temp. The outlet being hotter of course. BTW, what code?

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kcrowe

Multiple misfire was all they said.

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K.A. Crowe ;^)

johnnyrocks43110

very informative ty for this thread. i have similar problem this might also be my fix

kcrowe

So how could one shade tree mechanic like myself perform this test?

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K.A. Crowe ;^)