4.9 low rpm/light throttle stumbling/bucking
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HT4100 4.1, 4.5, 4.9 Discussion, 4.9 low rpm/light throttle stumbling/bucking in Cadillac Engine Technical Discussion; 1991 deville 4.9 171k miles (280,000km) Car ran great up until a couple weeks ago. Problem started with long cranking ...
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    caddeville's Avatar
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    4.9 low rpm/light throttle stumbling/bucking

    1991 deville 4.9 171k miles (280,000km) Car ran great up until a couple weeks ago. Problem started with long cranking before engine fired up. I replaced the distributor in my car with an ACardone reman one last year and again a month ago under warrnty thinking it would solve my long cranking issue. Car has literally new gas tank, pump, bosch injectors, MSD cap, rotor, coil, 8.5 mm conductor wires, platinum +4 plugs and accel ignition module (also have a GM one), new delco map sensor, egr valve is almost new and doesnt stick, no codes, tps adjusted, timing is fine, egs solenoid works, etc etc I've literally ive gone over the whole car while in my ownership. Now i'm having a problem where it'll stumble or bucks or whatever when i try to drive lightly, morning cold start it runs even shittier and does it even worse unless i drive it harder, then its ok. WTF i dunno what to do....other than low mileage junk yard original distributor to try. It still seems to idle normal, this problem is almost intermittent in a way......could the computer behind glovebox be f*cked?

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    Re: 4.9 low rpm/light throttle stumbling/bucking

    Have you eliminated a problem with the fuel pressure regulator? It's a prime suspect in hard starting/rough idling situations.

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    Re: 4.9 low rpm/light throttle stumbling/bucking

    well at idle i removed the vaccum hose...no fuel leaking...honestly the engine does basically almost the same stumbling at light throttle as if the hall effect switch went bad. The FPR im planning on replacing with LT1 fpr

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    Re: 4.9 low rpm/light throttle stumbling/bucking

    "Now i'm having a problem where it'll stumble or bucks or whatever when i try to drive lightly, morning cold start it runs even shittier and does it even worse unless i drive it harder, then its ok."

    I don't think it's the engine. I know you are going to roll your eyes, but it sounds like a transmission problem to me.

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    Re: 4.9 low rpm/light throttle stumbling/bucking

    I am drawing a blank but I don't think it is the distributor. If it was ignition related I think the plugs could be the weak link. You have a new EGR, did you also clean out the egr tubes? I would run some Techron in a tank of gas. How is the fuel mileage?

    On a side note, by buddy was working on a car last week and the parts store gave him the wrong spark plugs. He wouldn't have known if we didn't look it up to double check.

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    Re: 4.9 low rpm/light throttle stumbling/bucking

    Quote Originally Posted by caddeville View Post
    well at idle i removed the vaccum hose...no fuel leaking...honestly the engine does basically almost the same stumbling at light throttle as if the hall effect switch went bad. The FPR im planning on replacing with LT1 fpr
    If your FPR isn't leaking and has the correct pressure when engine is off, or key on as well the pressure is correct when idling, then it does not need to be replaced unless it is leaking from the vacuum port. I don't know what the difference is from a LT1. What is the LT1 in? You might also try taping a fuel pressure gage to the windshield to see what is is when your conditions are as you mentioned.

    Have you removed all of your ground connections, cleaned them and reinstalled?

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    Re: 4.9 low rpm/light throttle stumbling/bucking

    Stumble on tip-in is usually EGR related. Pull off the vacuum hose going from the solenoid to the valve and stick a screw or a golf-tee in the line, and see if that stops it.

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    Re: 4.9 low rpm/light throttle stumbling/bucking

    Sounds identical to my problem, I was considering bumping the ignition timing from 10 to 12 or 13 as I have the MSD cap, rotor, coil, and 8.5 performance wires too. FPR is fine, no leaks in the vacuum hose, Ilumina suggested the timing bump on his 4.9 high performance thread years ago.

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    Re: 4.9 low rpm/light throttle stumbling/bucking

    I'm waiting for some clown to drop in here and blame it on the Bosch +4's.

    Many good suggestions here. All I have to add is to consider the coolant temp sensor and/or MAT sensor. If either of them gets 'stuck' (failed) at a given temperature, it will cause problems. For instance, if your coolant sensor always reads say.. 197deg, you'll have a hell of a time cold starting it. Both sensors affect fueling and the coolant sensor influences timing and closed/open loop operation. Erroneous information from the sensors will cause all kinds of issues. The computer is only as good as the info it gets... You get the idea. If all else fails, disconnect the sensors(open circuit) and give it a try. With an 'open' the computer will automatically substitute default values.

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    Re: 4.9 low rpm/light throttle stumbling/bucking

    Replaced MAT sensor, in turn cured my long starting issue. However With gentle acceleration, we all know how sensitive the throttle is on these, the engine just cuts out, or if im sitting in the drive thru; engine begins to surge a bit and sometimes it'll stall. This problem I have noticed is intermittent. No fault codes.

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    Re: 4.9 low rpm/light throttle stumbling/bucking

    I read the recall regarding the main ground cable, alot of my symptoms are what GM posted. On my engine, half of the thread stud is broken off, my ground is on my engine mount and has been this way for a couple years. Problem started a month ago after I installed msd basics, new msd coil did not fix problem. But I'm gonna make a new ground spot on the block and go from there.....could be the computer also

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    Re: 4.9 low rpm/light throttle stumbling/bucking

    Did the replacement distributor come with a new harness in the distributor. I have read quite a few postings lately with harness problems.

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    Re: 4.9 low rpm/light throttle stumbling/bucking

    No it didn't, it's a reman ACardone brand. I got it replaced under warranty when the long starting issue began, it wasn't the distributor but the MAT sensor. This coming week I'll phone Cadillac dealer for a new harness. Cold mornings the car is normal due to moisture build up. Once everything gets hot then it begins cutting out with very light throttle. I switched back to 10w30 oil from rotella 15w40, engine runs Easier and it's noticeable. Anyways now there's a spark knock once it's hot at idle. I did pick up an extra computer.

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    Re: 4.9 low rpm/light throttle stumbling/bucking

    Ok, if you are going to replace the harness, do some shopping around. Stealership will be expensive. Here's a couple as an example if you want to price compare... Another thing I am surprised nobody has mentioned is a FSM. Tons of information and sometimes you can find a real bargain on ebay. Here's the harness below.

    http://www.amazon.com/ACDelco-104680...ype=automotive

    Here's one at rockauto.com

    http://www.rockauto.com/catalog/raframecatalog.php

    http://gm-partscenter.com/parts/1993...icleid=1025750

    Here's one that will knock your socks off price.

    http://parts.nalleygmc.com/showAssem...modelYear=1993

    Be careful, some of these places charge an excessive amount of shipping.

    http://www.autopartstomorrow.com/que...mit=Search+Now

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