slight hesitation at cold low-gear
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HT4100 4.1, 4.5, 4.9 Discussion, slight hesitation at cold low-gear in Cadillac Engine Technical Discussion; After my last major 4.1 tune-up there has been a slight hesitation when pulling out of my driveway. There is ...
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    ehall's Avatar
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    slight hesitation at cold low-gear

    After my last major 4.1 tune-up there has been a slight hesitation when pulling out of my driveway. There is also some periodic hesitation when coming from a stop light, but usually it's all gone by the time the engine warms up. There is no hesitation when sitting in Park in the driveway--it only happens when I put it under load, and mostly only when the engine is cold.

    This started happening after the last tune-up, which was pretty substantial and included things like GM top-end cleaner, new plugs and wires, and lots of other stuff. I'm pulling together parts for things that were missed like EGR valve, O2 sensor, breather element, and I'm planning on having the injectors tested and getting a leakdown test, so hopefully I'll get it with all that, but I'm curious what people think. My suspicion is that either (1) the condition existed before the tune-up but the engine running weak hid the problem before, or (2) deposits got knocked loose and are clogging something up. Any thoughts?

    Thanks

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    cadillac_al is offline Cadillac Owners Connoisseur
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    Re: slight hesitation at cold low-gear

    I'm stumped, maybe a plug wire not on tight?

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    Re: slight hesitation at cold low-gear

    My new theory is that the coil is bad. They replaced all the ignition components except the coil, so maybe the old one is dying.

    The FSM actually has a half-page diagnostics on hesitation that says "usually most severe when first trying to make the car move, as from a stop sign" so it's apparently not a freakish problem. The diagnostics say:

    -- check air cleaner damper door
    -- check EFE heater
    -- check fuel pressure and fuel quality
    -- check vacuum line to MAP sensor
    -- check spark plugs and wires
    -- check push rods, valves, cam lift (not gonna happen)
    -- check P.1.3 diagnostic
    -- check TPS
    -- check timing
    -- check EGR
    -- check injectors
    -- check for open HEI ground
    -- check canister purge
    -- check exhaust pipes

    I've already acquired the replacement EGR valve, O2 sensor, breather element, and PCV valve. I'm tempted to get a new coil and fuel filter for the heck of it. They swapped out the ISC motor (with new TPS) in the last tune-up (old one was failing), and they used good wires and plugs (just talked to the mechanic a few minutes ago). Might be bad injectors, so I've already decided to have them test that. I was hoping the leakdown test would help me figure out more about cylinder pressure but they don't have the machinery for it apparently. It could be the catalytic converter, given the age on this one and the super high emissions levels it used to deal with, but I'm not going to replace that for a couple of more months (going to test planned setup on my Jeep first).

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    Re: slight hesitation at cold low-gear

    I'm starting to think that it was just a problem with the THERMAC valve. I took my alternator off so I could put some spacers/washers in and move it over a bit, and made sure to reseat the THERMAC host really well afterwards. The hesitation has not really reappeared yet. Given that it's whole purpose is to choke off external air so it can pull in warm air off the exhaust, and given that the problem only started when the engine was first started (when the valve would be closed), it would make sense that the engine would stumble a bit if the valve or hose was not functioning correctly. I'll have it cleaned and lubricated when I get the rest of the parts installed, and we'll see if the problem reappears.

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    Re: slight hesitation at cold low-gear

    I rounded up AC Delco and Wix parts and had the mechanics replace EGR, coil, ignition module, fuel filter, breather filter and hose, and a few other things that were possibly related to a separate problem with high NOX emissions. I also had them advance the timing +3 degrees. The car is running better than it ever has, but I'm still getting a slight cough/stumble when coming off a stop with the engine cold. It's not happening as often though. I also had them check fuel pressure at the tank and injectors, TPS, Thermac valve operation, and everything else outside the valve covers. At this point I'm assuming that either the cat is clogged up (possible), or that there is some kind of simple problem with air or fuel delivery on acceleration.

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    awadecki is offline Cadillac Owners Enthusiast
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    Re: slight hesitation at cold low-gear

    Your EFE grid heater may not be working, or the ThermAC valve might not be operating properly. Make sure the heat stove is connected. Pull off the rubber accordion-like connector at the front of the air cleaner and make sure the valve is closing when you start the car and it's cold. When you shut the car off, it should open. In this weather, it may not even open when warm (it opens when the air inside the air cleaner hits 135 degrees).

    Also, clean out the MAP sensor port at the TBI with some carb cleaner or TBI cleaner. Just pull the hose off and squirt a lot in there.

    Injectors rarely go bad on the 4.x TBI engines.

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    Re: slight hesitation at cold low-gear

    Quote Originally Posted by awadecki View Post
    Your EFE grid heater may not be working, or the ThermAC valve might not be operating properly.
    Also, clean out the MAP sensor port at the TBI with some carb cleaner or TBI cleaner.
    Yeah, here's what I had done

    Replace the EGR valve and clean out the tubes.
    Replace the O2 sensor.
    Replace the crankcase breather element and hose.
    Replace the fuel filter, and check the fuel pressure at the pump and manifold.
    Replace the ignition coil and control module.
    Clean the MAP port on the throttle body and replace the tubing if needed.
    Clean the Thermac valve and check the hose.
    Check the plug wires for arcing.
    Check injector resistance.
    Check the TPS.

    Injectors rarely go bad on the 4.x TBI engines.
    They are consumables and are supposed to be replaced every 100k whether it's needed or not. Mine (still) haven't been replaced at 137k so I had them checked. I'm going to get them replaced anyway on the next service.

    Anyway, at this point the choking down on acceleration is something really simple.

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    carnut is offline Cadillac Owners Master
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    Re: slight hesitation at cold low-gear

    I would verify the TPS adjustment is correct. The TPS is simply a variable reostat that signals a longer injector spray as it rotates with the throttle angle. If the base setting is below spec then the delay in additional fuel would be felt as a hesitation. The TPS setting is easily readable thru on board diagnostics. It is either correct or incorrect.

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    awadecki is offline Cadillac Owners Enthusiast
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    Re: slight hesitation at cold low-gear

    I personally wouldn't replace the injectors. I don't see anywhere in the manual that it says these need replacement at 100k miles.

    I'd trust a Delco product with 100k miles on it over a new aftermarket part anyday. I've been stranded too many times doing preventative maintenance with NEW aftermarket, lifetime warranty parts that now I only buy Delco.

    In any case, I'd say your issue is minor. It may even be that the timing is set slightly below 10 degrees. When the motor is cold, the spark advance is less, so it will amplify this effect. Twist your distributor just a little bit to advance your timing and I'll bet that it'll go away.

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    Re: slight hesitation at cold low-gear

    I had them advance timing by +3

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    Re: slight hesitation at cold low-gear

    Couple of things... First, I added a bottle of Lucas Fuel Treatment to my fillup after getting the car back, and it has been getting progressively better. I don't have much hesitation at all right now (down to 9 gallons after 15 fill), and instead have a rapid "stepping" on acceleration where the RPM goes up a little, then a little more, then the engine "catches" and goes fine. Obviously something is clogged/dirty if the Lucas made that much of a difference. Okay, set that aside for a second.

    I have another issue which is that I frequently get E44 ENGINE LEAN error. I thought this was an O2 sensor issue, but I am still getting it after having the O2 sensor replaced. I now suspect that the E44 code is related to an actual lean condition, which would seem to tie-in with the above pretty well.

    So adding those two items I suspect a fuel delivery problem. The filter was just changed and the pressure tested fine at both ends, so I am guessing that one of the TBI injectors has a clog or a weak spring. Yes, most people don't replace them until they fail, and manufacturers do not usually require them to be changed, but they are mechanical and they wear out like everything else that suffers from entropy. They also collect varnish and the screens get clogged, which is not complete mechanical failure but will negatively affect performance (sometimes manifested as hesitation, as stated in the FSM and elsewhere). At this point I think it will be my best interest to have them cleaned or replaced with rebuilt injectors.

    BTW I ran the ECM diags while testing tonight, and the TPS readings look okay. The sensor reports about 6 degrees at idel on cold-start open-loop, and 2 degrees at idle after entering closed-loop. That seems about right to me but I don't know the desired numbers. I don't mind replacing the TPS and I probably should do so, but I don't think the problems are pointing that direction.

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    awadecki is offline Cadillac Owners Enthusiast
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    Re: slight hesitation at cold low-gear

    ehall: Sounds like you've pinpointed it. If you think it's an injector--and it may be--I'd go ahead and replace it. Check for vacuum leaks first, though.

    I wouldn't worry about the TPS. Your readings seem fine. As long as it registers progressively and you don't see any jumps in the readings, you're fine. The key is to get the TPS parameter to read 2 at idle in Drive (which is sounds like you have). Irrestpective of how it's set, the 4.x motor's ECU will always set the TPS reading to between 2.1-2.5 degrees on coastdown. When the car comes to a halt, the throttle angle will be based on engine RPMs, so you want these two values to match.

    In other words, if my TPS was set incorrectly, and say, read 0 degrees at idle, the ECU would extend the ISC and effectively powerbrake the engine until you came to a stop at idle.

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