marie Posted September 19, 2012 Share Posted September 19, 2012 I have and 88, 4.0, standard. I put a new engine in 2 years ago (pulled it all by my tiny self, hence the self portrait in the left corner). This Comanche is truly a labor of love. It was running great afterthe new engine, but the honeymoon is over. I'm having trouble diagnosing the latest challenge. It started to kind of hiccup and/or stall while driving one night. Then completely stalled on the highway and would not restart. It had plenty of power to turn over but wouldn't completely start, though it valiantly tried. I guessed fuel issues so I had my son bring me a fuel filter... it was worth a try. I put it on and it started. Wow, did I actually fix it on the side of the road??? Well, no. It drove home then started to do it again the next day. I was almost on E, so I got gas and suddenly it drove great! It stayed running for a couple of weeks, so I figured I had gotten bad gas. Well, no, again!! it started doing it all over, and I had just gotten gas at Shell, the gas couldn't have been that bad, right? I did some goggling and tried replacing the MAP. Nope, didn't fix a thing. It barely wants to idle and the idle is all over the place like it's trying to figure out how much gas it needs to stay running, and will eventually die. Plus, it is very rough sounding, almost like a timing issue, but I don’t think that’s it. I bought a fuel pressure gauge and goggling tells me it should read 39-41 psi, providing I have MPI (I think I do?) I will describe what the fuel pressure gauge did because if I had to guess, I would have called in a priest for an exorcism. One friend actually suggested I park my truck in a church parking lot over night to see if that would help. Anyway, the gauge violently bounced from 35-40psi while it tried to idle, even after it had warmed up. I had my son try to hold the rpms at about 1500 and the gauge would drop to 30psi and be slightly more stable but not by much. Even when the throttle was held at 1500 rpms, there was still a lot of fluctuation like it was confused and unstable. Also, I have checked all the vacuum hoses and they are all in place as far as I can tell. My gut tells me it's some kind of sensor or vacuum issue. From what I have read here in some of the forums, I think I need to do a vacuum test next and I'm guessing again (I do a lot of that) that I need to buy another gauge. There is a lot of talk in these forums about valves and sensors like: Exhaust Gas Recirculation (EGR) valves, Idle Air Control (IAC), Throttle Position Sensor (TPS) and the Electronic Control Module (ECM). I hope it's not the computer $$$! I don't know where to start with these, is there an order to go in and can you test or clean them, maybe even bypass one or two? The last car I did a lot of work on was a 72 Cutlass with a 350 rocket, it was so simple, sigh... I could sit in the engine compartment with plenty of room for beer and tools. There wasn’t much in the way of electronics or vacuums. So if anyone can point me in the right direction with out kicking me in the a$$, I would be grateful. Many thanks in advance! Link to comment Share on other sites More sharing options...
1987Comanche Posted September 19, 2012 Share Posted September 19, 2012 I'd look at the fuel pump next. It sounds like the pump overheats and is shutting down on you. Can you make it buck, surge, etc or does it happen randomly? The next time it does it can you verify fuel pressure & flow before the engine cools off? Link to comment Share on other sites More sharing options...
cruiser54 Posted September 19, 2012 Share Posted September 19, 2012 Fuel pressure should be rock solid at 39 PSI at idle with the vacuum hose disconnected from the fuel pressure regulator and 31 PSI with it plugged in. Don't go chasing sensors just yet. Do you have a voltmeter and know how to use it? Link to comment Share on other sites More sharing options...
marie Posted September 19, 2012 Author Share Posted September 19, 2012 @ 1987 Comanche, the scenario I described with the fuel pressure gauge included the engine being warm. So, the pressure gauge reading didn't change with the engine warm. I will try it again tonight to be sure. It has happened randomly, I can't force it to happen. It got progressively worse and is undrivable now. I could try to drive it around the block a few times, it would require a lot of nursing it along and leave the pressure gauge attached at the rail to test it as you described. How do I check the flow? @ Cruiser54, I did not check the fuel pressure gauge with the vacuum hose disconnected from the pressure regulator. I will do that tonight when I recheck the reading. I don't have a voltmeter and have never used one, but I am willing to get one and engage my brain if you can give me a few hints. FYI, at idle the throttle is all over the place. It goes from almost stalling to revving and will die if you don't tap the gas pedal when it starts to die out. thanks for the replies! Link to comment Share on other sites More sharing options...
jimoshel Posted September 19, 2012 Share Posted September 19, 2012 Try the basics first. Is there crud in the tank? It is possible to remove the fuel pump, sending unit and use a light and mirror to look into the tank. However I would recommend doing it right. Drop the tank, clean it and check pump and filter while it's out. Eliminate the easy (IE, Cheap) before you start throwing money at it. Link to comment Share on other sites More sharing options...
cruiser54 Posted September 19, 2012 Share Posted September 19, 2012 Check the pressure as sugested first. I'm concerned about this. You might have a torn/split hose in between the pump in the tank and the top of the unit. That's easy. Buy a $15 to $20 digital multimeter and we can talk you through anything electrical. Link to comment Share on other sites More sharing options...
comanche1989 Posted September 19, 2012 Share Posted September 19, 2012 When mine was doing that I replaced the egr valve and the tps. Idled like a champ! But of course go through the simpler and cheaper fixes before buying parts! Link to comment Share on other sites More sharing options...
marie Posted September 19, 2012 Author Share Posted September 19, 2012 Thanks all! I will check the fuel pressure as suggested by Cruiser54 when I get home from work. I will report the results asap. I will also get a multimeter, I have always wanted to learn how to use one, I've jsut been lazy. @Cruiser54, so if there is a split or tear in the fuel line as you suggest, would that cause it to race and drop because the sensors can't figure out the fluctuation in fuel? Hence checking the pressure without the regulator? With the type of fuel tank I have, the fuel pump lines are in front and not under the bed. Am I correct in thinking I don't actually have to drop the fuel tank to get to the fuel pump? Link to comment Share on other sites More sharing options...
cruiser54 Posted September 19, 2012 Share Posted September 19, 2012 It's hard to pinpoint anything right now until we find out about the fuel pressure. If the fuel pump pulls fuel and air alternately, there could be a fluctuation in idle, not because of sensor readings, but because of the fuel pressure fluctuation. With less than 1/2 tank of fuel, you won't have to drop the tank. Link to comment Share on other sites More sharing options...
marie Posted September 20, 2012 Author Share Posted September 20, 2012 Here's the report: At cold start THIS time it idled with a pressure reading of 34 psi and held fairly still, almost no vibration of the needle at all. With the fuel pressure regulator vacuum hose off it jumped to 40 psi and the needle was rock solid. It stayed the same with the engine warm with the tiniest bit of needle vibration here and there at 34 psi. When warm and the vacuum hose off it did the exact same thing and jumped to a very still 40 psi. I turned the engine off a few times and restarted hoping to get it to run as rough as it did the last time, but to no avail. It still sounds rough at idle, just not stalling out at the moment. I can hear the fuel pump working. I did purchase a multimeter, I will google some user instructions and bone up on it. Here is another tidbit and maybe just another problem all together.... my exhaust smells bad (rich) and it's blowing some black, like it's burning oil. The engine is only 2 years old and I take care of it and change the oil regularly. I also just checked the oil while I was out there and it is perfectly full. It can't possible be burning oil can it? again, thanks in advance Link to comment Share on other sites More sharing options...
cruiser54 Posted September 20, 2012 Share Posted September 20, 2012 Check the hose/piping from the throttle body to the MAP sensor located on the firewall directly behind the valve cover. Is it plugged into the throttle body tightly? Are there no crimps, cracks, melted spots in the line? Link to comment Share on other sites More sharing options...
Jeep Driver Posted September 20, 2012 Share Posted September 20, 2012 Pull your plugs and examine them. One at a time of course. Link to comment Share on other sites More sharing options...
marie Posted September 20, 2012 Author Share Posted September 20, 2012 Yes, the line from the MAP to the throttle body is good/clean and tight in the rubber plug it fits into. I installed a new MAP two days ago when it first became un-drivable and was curious about something. The rubber plug on the throttle body that the MAP line fits into has two vertical holes. The MAP line fits into the lower one and the upper one had nothing plugged into it, and it doesn't look like it ever did. Is that normal? It is dark out now so I will have to wait until tomorrow to double check all the vacuum lines. I don't have a garage or big lights to work after dark. :( I did look them over the other day while doing the MAP and they all looked healthy and in place, but I will look closer tomorrow. If all the vacuums are good, what is next? Link to comment Share on other sites More sharing options...
marie Posted September 20, 2012 Author Share Posted September 20, 2012 ps, I will have to do the plugs tomorrow in the light too. Link to comment Share on other sites More sharing options...
cruiser54 Posted September 20, 2012 Share Posted September 20, 2012 That is normal where the line plugs into the throttle body. Link to comment Share on other sites More sharing options...
marie Posted September 20, 2012 Author Share Posted September 20, 2012 I will check the plugs and wires and make sure all is well with that. I suppose if the plugs need changing that could make it run rough and contaminate the exhaust, right? But would that make it stall out like it has been? Sorry, I know I'm full of questions, the logical side of my brain is on fire. Link to comment Share on other sites More sharing options...
Jeep Driver Posted September 20, 2012 Share Posted September 20, 2012 I will check the plugs and wires and make sure all is well with that. I suppose if the plugs need changing that could make it run rough and contaminate the exhaust, right? But would that make it stall out like it has been? Sorry, I know I'm full of questions, the logical side of my brain is on fire. You said the exhaust smells rich that that tells me that you are not burning off the gas. Also you smell oil or oil being burned off. If all 6 plugs look tan and all six look similar then all cylinders are firing. If one looks fouled, covered with oil, smell of gas...........either the wire is bad or the plug is bad or you have a mechanical problem. The plugs will tell you whats going on inside. Link to comment Share on other sites More sharing options...
marie Posted September 20, 2012 Author Share Posted September 20, 2012 Yeah it smells rich and is a little black, though I am not an expert at that. Yes, the spark plugs are like the windows to the internal combustion as eyes are to the soul. (auto poetry??) I should have thought to check them earlier. I think my brain was focusing on sensors and external valves. So, the report I gave about the fuel psi, is that in the normal range? I thought the psi was supposed to go down when I took the vacuum line off the regulator? Link to comment Share on other sites More sharing options...
Jeep Driver Posted September 20, 2012 Share Posted September 20, 2012 BTW have you checked for condensaton in the distributor cap? Link to comment Share on other sites More sharing options...
marie Posted September 20, 2012 Author Share Posted September 20, 2012 I live in the desert, it hasn't rained in several months, but I will check it. Link to comment Share on other sites More sharing options...
cruiser54 Posted September 20, 2012 Share Posted September 20, 2012 The fuel pressure rises when the hose is removed from the fuel pressure regulator. What that regulator actually does, is provide 39 PSI in the rail and that's plenty to run at throttle. It reduces the fuel pressure for idle. Link to comment Share on other sites More sharing options...
marie Posted September 20, 2012 Author Share Posted September 20, 2012 So, the pump checks out ok, right? And it's obvious that the choke/stall problem is intermittent. Let's see what I can get dialed in after work today. It's a process of elimination now. Link to comment Share on other sites More sharing options...
cruiser54 Posted September 20, 2012 Share Posted September 20, 2012 Process of elimination is good. Have you ever cleaned the throttle body and Idle Air Controller also known as IAC? Link to comment Share on other sites More sharing options...
marie Posted September 20, 2012 Author Share Posted September 20, 2012 Well, I have cleaned and rebuilt carborators before but not this kind of throttle body. I just looked a picture of it and it's that little doohicky that comes out the side of the throttle housing and has a little stem that rests inside. I can clean it with some carborator fluid and a tooth brush then let it air dry for a bit, right? I will pick some up with the spark plugs on the way home. Can one clean the EGR in a similar fashion or is that strictly a replacement item? Thanks! Link to comment Share on other sites More sharing options...
cruiser54 Posted September 20, 2012 Share Posted September 20, 2012 -------------------------------------------------------------------- The Idle Air Control (IAC) is mounted on the back of the throttle body. (front for 87-90) The valve controls the idle speed of the engine by controlling the amount of air flowing through the air control passage. It consists of a stepper motor that moves a pintle shaped plunger in and out of the air control passage. When the valve plunger is moved in, the air control passage flows more air which raises the idle speed. When the valve plunger is moved out, the air control passage flows less air which lowers the idle speed. Over time and miles, the IAC can get carboned up which can have an adverse affect on idle quality. Cleaning the IAC may restore proper function and is an easy procedure to perform and good preventive maintenance so it is never a bad idea. CLEANING THE JEEP 4.0 IDLE AIR CONTROL Remove the air filter cover, associated hoses and the rubber boot that goes from the air filter cover to the throttle body. Remove the IAC with a torx driver (2 bolts; one can be kind of hard to get to) “Gently” wiggle out the IAC from the throttle body. Gasket on the IAC can be re-used if it is not damaged Clean the IAC with a spray can of throttle body cleaner; inexpensive and available at any place that sells auto parts. Throttle body cleaner is recommended rather than carburetor cleaner as it is less harsh, safe for throttle body coatings and is best for this task. Use cleaner, a rag and a toothbrush and or Q-Tips. Be gentle; don’t twist or pull on the pintle that protrudes from the IAC as it is fragile and you could damage it. Thoroughly spray clean and flush where the IAC seats in the throttle body with the same spray cleaner It is also a good idea to clean the entire throttle body itself, the butterfly valve inside of the throttle body and all associated linkage as long as you have things disassembled Link to comment Share on other sites More sharing options...
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