Dando Posted June 10, 2016 Share Posted June 10, 2016 1990 MJ 4.0 aw4 231. So let me start with some back story. This past November the head gasket crapped out. I pulled the head and replaced the gasket and in the process the intake/exhaust gasket. Since then it has an issue. So when driving off the line it feels and operates normally. When at speed or say you approach a hill or need to quickly speed up, when you push further down on the throttle the truck actually drops down 3-400rpms and will have zero acceleration until the top of the hill or I go over 3000rpm. Especially noticeable on the highway. So far I have checked for intake and vac leaks with propane and have found no leaks using this method. I have verified the tps is set properly. New IAC. I adjusted the TV cable. New O2 sensor. EGR tests good. Fuel pump was new in October and still has perfect psi. Thoughts? Link to comment Share on other sites More sharing options...
HOrnbrod Posted June 10, 2016 Share Posted June 10, 2016 Do a compression test. Link to comment Share on other sites More sharing options...
Dando Posted June 10, 2016 Author Share Posted June 10, 2016 Ok I have never used a compression tester before. I will buy one tomorrow and give it a shot Link to comment Share on other sites More sharing options...
gogmorgo Posted June 10, 2016 Share Posted June 10, 2016 Are you saying it's not down-shifting if you push the gas down when going uphill? For me cruising is just above 2000 rpm in overdrive, come to a hill and push the gas it downshifts, usually goes to say 3200 rpm, and then if it's a long hill drops down to 2700 or so when the torque converter locks. If I don't get on the gas too hard, it just unlocks the torque converter without downshifting, and revs up to around 2500rpm or so. Link to comment Share on other sites More sharing options...
Dando Posted June 10, 2016 Author Share Posted June 10, 2016 Are you saying it's not down-shifting if you push the gas down when going uphill? For me cruising is just above 2000 rpm in overdrive, come to a hill and push the gas it downshifts, usually goes to say 3200 rpm, and then if it's a long hill drops down to 2700 or so when the torque converter locks. If I don't get on the gas too hard, it just unlocks the torque converter without downshifting, and revs up to around 2500rpm or so. Not exactly. Even when it downshifts I have no acceleration till I hit over 3k rpm Link to comment Share on other sites More sharing options...
Nofeartoys Posted June 12, 2016 Share Posted June 12, 2016 Also is this issue worse when warmed up ,compared to first driving? Possible plugged exhaust i.e. Cat. I had debris in my fuel rail and my MJ would idle fine and have power at start but limited power driving and when pushing throttle down seem to lose power. Fuel pressure was good but if I watch guage and revved engine hard I would see pressure drop. Maybe drive with fuel gauge taped to windshield to watch pressure..( fuel pressure is only half. Fuel volume is other part) Link to comment Share on other sites More sharing options...
ParadiseMJ Posted June 12, 2016 Share Posted June 12, 2016 Do a vacuum check for a clogged catalytic converter. When my cat crapped out it "leveled out" at about 2800 rpm, anything over that it just would not do. I live in the hills, so it was a problem. Link to comment Share on other sites More sharing options...
HOrnbrod Posted June 12, 2016 Share Posted June 12, 2016 The way I read it, he replaced the head gasket (for unknown reasons) then his problems started. Clogged cat, fuel pressure etc etc. blah blah would have already been there. A compression test is first in logical order in my humble opinion to verify the install. OP, what were the symptoms as to knowing "the head gasket crapped out"? Link to comment Share on other sites More sharing options...
Dando Posted June 12, 2016 Author Share Posted June 12, 2016 I replaced the head gasket because I had a thermostat that failed to open and the engine overheated, I had steam coming out from where the head meets the block and coolant too so I changed it. Link to comment Share on other sites More sharing options...
Dando Posted June 12, 2016 Author Share Posted June 12, 2016 Also it can't be the cat because I don't have one. And I still didn't have a compression tester but that is the rote I plan to take Link to comment Share on other sites More sharing options...
Dando Posted June 14, 2016 Author Share Posted June 14, 2016 Ok so I was able to perform the compression test and from what I have been reading it doesn't seem like compression is my problem. My results varied among the cylinders which a range of 120-145psi Link to comment Share on other sites More sharing options...
Zebvance Posted June 14, 2016 Share Posted June 14, 2016 Did you resurface the head and have it mic'ed for warpage or cracks before you reused it.... When replacing a head gasket because it has failed and the engine has overheated, replacing it and not checking and resurfacing the head before you put a new head gasket is about as good as brushing your teeth without tooth paste... Link to comment Share on other sites More sharing options...
Dando Posted June 14, 2016 Author Share Posted June 14, 2016 Well then I didn't use any toothpaste. I was in a real hurry. Only checked the head mating surfaces with a straight edge. By if the head were warped or cracked wouldn't it run bad all the time? And not just on hard acceleration? Link to comment Share on other sites More sharing options...
Zebvance Posted June 14, 2016 Share Posted June 14, 2016 Well then I didn't use any toothpaste. I was in a real hurry. Only checked the head mating surfaces with a straight edge. By if the head were warped or cracked wouldn't it run bad all the time? And not just on hard acceleration? yes and no. When the motor is cold there may not be a air leak via hair line crack or warped mating surface, but when the engine is warm the the metal could expand or contract in certain places and open up a leak that wasn't there when It was 200 degrees cooler. Also, when you are giving the truck "gas" the engine is gasping for air in the head to accommodate the right air to fuel mixture in the combustion chamber, sucking it through seepage points that do not show them self's until a certain negative pressure is reached in the intake/head. However, I would also think that it would be shown in a compression test but I have seen a lot more odd things happen. Link to comment Share on other sites More sharing options...
Dando Posted June 14, 2016 Author Share Posted June 14, 2016 Can anyone provide a sensor to test or some other tests that could be done before I decide to replace the head? I will be doing work to the truck in preparation for camping this weekend when I get home tonight. I can update tests results then. I am trying to figure this out. I plan to run through the tests in cruisers tips again tonight Link to comment Share on other sites More sharing options...
Zebvance Posted June 14, 2016 Share Posted June 14, 2016 Can anyone provide a sensor to test or some other tests that could be done before I decide to replace the head? I will be doing work to the truck in preparation for camping this weekend when I get home tonight. I can update tests results then. I am trying to figure this out. I plan to run through the tests in cruisers tips again tonight That would be something I'm not knowledgeable on. hopefully someone else will chime in. Link to comment Share on other sites More sharing options...
mik666 Posted June 14, 2016 Share Posted June 14, 2016 did you check the compression with the engine warmed up? maybe that will tell you if something is amiss when warm, as if the cylinders are all about the same that would suggest it is something else Link to comment Share on other sites More sharing options...
Dando Posted June 14, 2016 Author Share Posted June 14, 2016 I tested compression both warm and cold. Both times results were similar with no apparent issues Link to comment Share on other sites More sharing options...
Dando Posted June 20, 2016 Author Share Posted June 20, 2016 Ok so today while driving the engine got much louder all of a sudden. Seemed to actually improve the throttle response slightly and helped me keep speed on hills somewhat. The problem still exists but not to as great an extent. When I arrived home I crawled underneath and low and behold the muffler is beginning to seperate. I plan to order a new one and replace but will remove it tonight. Being I do not have a cat and someone suggested clogged cat, could my muffler have been rusted and clogged up to an extent and if so would it cause similar symptoms as a clogged catalytic? Link to comment Share on other sites More sharing options...
gogmorgo Posted June 22, 2016 Share Posted June 22, 2016 What kind of a muffler are you running? Cats can clog because they have a whole bunch of very narrow passages that easily fill up with soot. Most mufflers are just baffled, so you'd be plugging up 2" holes or something like that. Did you check to see if the tailpipe got crushed somewhere after the muffler? Link to comment Share on other sites More sharing options...
Dando Posted June 22, 2016 Author Share Posted June 22, 2016 Not sure what muffler was on the truck, some kind of stock version. To update this thread. I did cut the muffler off. With the muffler removed the truck has the power to pass on the highway and doesn't seem to be losing rpms in hills. Small hesitation at random times now but I think that is due to the open headers basically telling the O2 to riches up the mixture. Any thoughts? Also any recommendations on mufflers, no need to stay stock just want something affordable and with good sound/performance. Link to comment Share on other sites More sharing options...
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