Hesitation at 1200-1600 rpm 1995 Grand Am GT 3100

[JoeMechanic]

Now I have a bad hesitation problem associated with my rough idle and stalling problem.It seems the colder the weather the worse it is.Can Ignition Wires cause these problems?...I am pretty much at a loss of what to do next.I have replaced plugs,FF,Intake Gaskets,Valve cover Gaskets,Fresh oil change.This is a bone yard engine that was just installed and was tested prior to purchase and ran fine in the car it came out of which was a 1996 Buick Skylark 3100 M engine also.It runs as though one cylinder isn't firing I used the wires that came with the engine and a couple from the old engine..they looked good so I installed them.Also sometimes while driving around 40 mph it kicks in and takes off and seems to be running on all 6..could be wires?..I am replacing them today and go from there..any suggestions would be appreciated ..thanks in advance,joe.

[IsaacHayes]

Didn't you have a thread already about this???

[JoeMechanic]

Yes I did..I am new give me a break

I thought by putting a new thread I would get quicker responses..I am just frustrated with this junk I am working on.

[gpse3400]

Try new wires..also the Igniton control module might be in question too. If your old one worked, I'd swap it over, along with the coils.

[JoeMechanic]

Been there done that

New Wires installed today..it helped some..swapped known good ICM a long time ago.The way it is running leads me to believe that one cylinder has a burnt valve or something wrong with it.This engine has 140,000 miles on it and very well could be a bad motor.The only logical thing left to do is a compression test which will reveal what I suspect or not.Now to look for the compression specs on here,Joe.

[gpse3400]

if you are running about 150 psi then you should be ok.

[JoeMechanic]

Alldata Online has the compression test spec at

100 PSI with no more than 70% difference between cylinders which would make it no lower than 70 psi difference.Thats fricken low if you ask me I have never seen a spec that passes if its 70 psi..lol.

Also the fuel Pressure specs out at 40.5-47 PSI Ignition ON Engine Off.

Tomorrow I will be testing Pressure and Compression to get to the bottom of this problem child.I have a fuel pressure gauge that screws onto the fuel rail so that will be easy..I don't have a compression tester but I do have a friend that owns a garage 2 miles from my house and would lend me his snap on tester.I can't wait to figure this rascal out and let all of you know what a great mechanic I really am..lol.

[JoeMechanic]

Correction..Last reply is wrong..here is exactally

what Alldata online has listed:Compression Check


The lowest reading cylinder should not be less than 70% of the highest and no cylinder reading should be less than 689kpa(100 psi).Perform compression test with engine at normal operating temperature,spark plugs removed and throttle wide open.

[JoeMechanic]

no wonder I never saw a 70 psi reading..THERE ISN'T ANY unless its a lawnmower engine duh..lol..I really have to pay closer attention to what I read.The above post is right on the money..its 70% of the highest reading and no reading should fall below 100 PSI is how it was stated.

[JoeMechanic]

Fuel Pressure is at 36 psi Ignition on Engine off.It is supposed to be between 41-47 psi according to Alldata..I can't even hear the pump cycle when I turn on the Ignition switch.Replacing the pump will most likely solve the hesitation and stalling..now to do a compression test and go from there..if bad compression in one or two cylinders what do I look for next..burnt valve or bad rings?...maybe I'll drive it through the front Door of the wrecking yards offices?.They assured me its a good motor and gave me a 100 day warranty..its been about 45-65 day's since purchase but man it was alot of work putting this motor in from the top(no car hoist so I had to pull it with cherry picker that I had).I still have the heads off of the original motor and they just need valve job and cleaned up and checked for straightness they are good heads.

[IsaacHayes]

When you floor it does it move ok? If it's the pump then wouldn't that affect it when it's under load / WOT?

Here is a measurement of my system: 38 psi w/vacuum, and 46 psi without to the FPR, engine running.

Get a compression test before you throw anymore parts at it. You have verified all ignition components right? Did you swap injectors yet?

Oh and the CPS 7x sensor in the block when mine went it would cut out while cruising but when you floored it you couldn't tell. Might want to swap the CPS with one from the other motor just to get that out of the equation. It's a bit hard to get to with the engine in the car but I have done it several times on my car. Just have to do it by feel using a 10mm deep dish socket on 1/4" ratchet. The damn CPS has cost me lots of money in parts before. Don't get an autozone brand one though whatever you do stay away from their electronics..

[JoeMechanic]

Fuel Pump is going bad..this one has 40 psi engine

running..unplug FPR vacuum hose and its 35psi..thats normal but 40 psi is too low and I can't even hear the pump cycle on when turning ignition to on..even when engine is running I can't hear a buzz.So I know its crapping out but my main focus right now is why it shakes like its running on 5.Did compression test just now and here are the results..please decipher these results and tell me what you think.Please remember that this is a bone yard engine with about 140,000 miles on it.guaranteed for 100 day's by yard..so its still under warranty.

Cylinders in order: #1 (200 PSI) #3 (190 PSI) #5 (200 PSI) Rear bank


#2 (190 PSI) #4 (195 PSI) #6 (190 PSI) Front bank

[JoeMechanic]

Next is Leak Down test..have to buy tester first

I almost bought a leak down tester from the local harbor freight tools..it is about $30..I bought the quick disconnect compression tester instead for $14.99 with blow molded case..worked great!.I am leaning towards a valve not opening properly due to a possible worn out cam lobe or lobes..what do you think?.Why is the compression so high on the rear back..200 psi is kinda high no?..thanks for all your help with my problem child,Joe.

[IsaacHayes]

I'd say thats good #'s. Those are high! Which would be good.

Wait so when the FPR has no vacuum it's 35psi only? or you mean with vacuum tube on....

EDIT: Try the CPS swap, then try swaping injectors...

[JoeMechanic]

with Vacuum hose connected its 35psi

when I goose it it does jump up to about 41 so I know its working fine.When I unplug the vacuum hose it is steady at about 40..same as Ignition ON Engine OFF test.My other tester was reading 36 psi but new tester is around 40 shich is still below spec.Spec is between 40.5 and 47 psi..not crazy Low but still Low.I believe that is the cause of my hesitation but the shaking is unbelievable...It is definately running on 5 and now I am scratching my head.I did notice when I removed the plugs that #6 cylinder the plug looked clean as if it wasn't firing compared to the other plugs.They are new with approximately 100 miles on them from test drives.I am thinking that possibly the Camshaft Lobes on number 6 cylinder for intake and or exhaust valves are worn..what do you think?.