Potatowalker

Hi everyone, I have been working on tracking down the cause of my cold start issues. I'll describe my symptoms, what I've determined so far. Then I'll ask the big questions. I have a 96 4.0 auto with 190K.

1. Jeep starts and runs fine at normal op. temp. of 210ish.
2. Jeep will not start when engine is cold unless I feather the gas pedal. If I take my foot off the gas, it stalls right away, seems like it looses air.
3. Tach died about 6 months ago, figured bad connection, left it.
4. Voltmeter in dash cluster started reading high about a month ago, seems to be only a part-time issue, not always there. Doesn't seem to be related to charge as it can read high when i start it then it goes down to normal after a bit, but only sometimes.
5. Fuel pump primes when key turned to on.
6. CEL has never come on, but works normal when key turned to on, i.e. it comes on, then goes out after a few.
7. If I keep my foot on gas after getting it to start, let it warm up a minute or two, then turn the key off - it will start if I let it sit a few minutes before trying to start again. When I do this it starts up right away and idles fine, running like it should.
8. Batter is less than a year old and tests at 12V after sitting overnight. Have not had a load-test done since it's new and high amp.

I've taken my TB apart, cleaned, removed IAC, cleaned. I noticed my CCV valve is "repaired" by PO with some kind of tape. I plan on replacing that asap along with the other breather valve at the front and probably just gonna do a valve cover seal and the hoses all at once. It's leaking a little around the cover.

Anyone have any suggestions? after reading posts on here, I'm guessing it's the IAC needs replaced, there is a bad ground somewhere (gonna do all new ground wires this week), or my ECU is going bad. Figured I'd start with the ground on the back of the engine, then the IAC replace and then try a new ECU if that doesn't fix it. I do not have access to diagnostic tools other than a DMM. Can't get the CEL to do the code-flashy thing either.

Does anyone know where the Tachometer gets its data? Would my tach malfunction indicate a possible ECU failure when considered with my other symptoms? All my other gauges seem to read correctly, except maybe the volts from time to time.

It will always start if I feather the gas pedal and it never cuts out while driving so I'm pretty sure it's not CPS. Really seems like lack of air to me.

Don't wanna throw parts around if I don't have to, so figured I'd ask y'all! Thanks in advance for any help!

tjwalker

Your problem is not the crank sensor or they generally don't run at all, you won't have spark or fuel to the injectors if that sensor is bad.

And hold off on that computer. This almost assuredly is not a computer problem. Replacing the computer should be a "diagnosis of exclusion", which means that everything else needs to be ruled out before replacing it.

1. Always review tuneup hardware when symptomatic. Fresh plugs, plug wires, distributor cap, rotor.

2. You need to get a fuel pressure gauge and do some fuel testing. You could be suffering from "pressure drop" while the engine sits and it is taking a while to build up pressure. Only way to rule that in or out is with a gauge (can be rented). See what the pressure is before you start that cold engine, then what it is when it first starts and what it eventually climbs to. This is one of the first things that an ASE technician would do.

3. The throttle position sensor is also in play. They can be tested.

Freshening grounds is never a bad idea and I highly recommend it to everyone, but I don't think that this is your problem.

Good luck and let us know what you find. But use a "testing based" strategy. Don't throw parts at this on a hunch. It is not efficient and becomes expensive in a hurry!

Potatowalker

Thanks for your input, I'm somewhat new to working on engines and on a pretty limited budget. I'm not afraid of a challenge and I'd much rather figure out what's wrong before making any decisive purchases! I've got a multimeter coming so I can start testing a few things and I'll look into the fuel pressure gauge. I just did a tune up in December and it was problematic prior to that; and when I did the fuel filter with the tune up there seemed to be good pressure on the fuel rail, but i didn't measure it. The only reason I was thinking it was the computer was because the tach is all wrong and the voltmeter started reading high when it was acting up. Definitely gonna be sure that's what it is before buying one of those though. I'll be sure to update when I have a chance to do some more investigating. Thanks again!

93gc40

Issues with the tach and voltmeter sound more like bad gauges, than anything else. If you changed the fuel filter good, You are on the right tract. have pressure checked, cause you either have dirty/leaky injector or a failing pump.

Potatowalker

So i purchased a fuel pressure test kit from Slamazon for about $60:
Way better than the only local option which was to order a $180 kit that had a fancy plastic case and less fittings from Car Quest. No one within 100miles had anything to borrow/rent and at that price is about the same as paying a shop to test it for me. I lubed the o-rings it came with prior to use and it hasn't leaked a drop on me yet. The gauge threads came from the factory with teflon on them. I'm impressed so far with this set - it even came with a banjo fitting so I can help my brother with his Yota!

Anyway, hooked it up after letting the jeep sit overnight which pretty much always caused the problem. Gauge read zero after sitting 9hrs. Turned ignition to prime fuel pump, gauge jumped to 20psi, primed pump again, gauge went up to 40psi. Turned key to start, fired right up, idled fine without any further input from me. This is weird because I tried the "pump prime" before to see if maybe it was a bad check-valve and it didn't seem to have much effect? I'm pretty sure after thinking about this that this would mean a leaky injector as opposed to the check-valve.

After I get home tonight I'll hook up the gauge again and put a clamp on my fuel line in front of the filter where it's rubber hose to see if it's the valve or injectors. I have yet to find any leaks or notice any strong smells of gas anywhere.

I'm still not convinced this isn't more than one thing (bad TPS, etc), since it has 190K and looks to have been somewhat neglected by the PO. But I'll start with the obvious for now! I'll be sure to post more as I figure more out! Thanks for your input everyone!

VTJeep

i was gonna say, TPS, IAC valve, or (worst case) your o2 sensors...

also get it on a code reader as there might be a code stored in there that didnt trip the CEL

jeepmudmaker

have u checked the ignition control moduale or crank sensor those are two of the biggest reason for hard start when cold

Potatowalker

Yea, I'm still not ruling out the TPS or IAC. I cleaned the IAC real good, but am not positive it is actually actuating properly. I have yet to test the TPS since I found some kind of leak-down in my fuel pressure.

I've determined that the fuel pressure is zero on the rail after letting is sit overnight. After getting it started, it runs at a steady 49psi. Once I shut it off, it slowly loses pressure - after an hour of sitting, after reading 49psi at shut-down it will drop to 20psi on the rail. I've primed it back up to 49psi by running it, and now have it shut off and have clamped the rubber fuel hose behind the fuel filter to start figuring out where the loss of pressure is occurring.

After twenty minutes clamped and off it seems to be holding pressure where without the clamp on the line it had already leaked down to about 30psi, so at this point problem #1 (and hopefully the only problem) is a loss of pressure in the fuel pump or pressure regulator or bad check valve.

If I understand correctly the pump, check valve, and pressure regulator are all part of one assembly and by replacing this I should be able to eliminate my pressure loss? Gonna make sure by leaving it clamped overnight to make sure it's not just a real slow leak in the injectors, but it seems like I've narrowed it down to the fuel pump/assembly...

I'm sure there are good quality and cheap-o fuel pump assemblies - anyone have any experience with what brands will last???

I may also just install an in-line check valve until the snow melts and things are easier to clean and work on...

tjwalker

You have done a good job of diagnosing. Always good to see! Your results clearly show that you are leaking from the side to the "rear" of the clamping, which means you almost assuredly have a check valve problem.

Having a failing check valve doesn't mean that the fuel pump itself is bad (the fact that you are running at 49 psi shows you that).

Installing a brand new assembly should set you up. I would highly recommend purchasing a Bosch fuel pump assembly. Under no circumstances should you buy an Airtex unit. Their out of the box failure rate is through the roof. Delphi is okay too, but Bosch rules with fuel pumps.

Skip the inline check valve. Try the "poor mans prime". Along with allowing you to get a quicker start until you get this resolved, it also helps in confirming your diagnosis.

1. Turn the key to the ON position. Do not crank the engine. The fuel pump will energize and run for about 2 seconds.

2. Turn the key to the OFF position

3. Repeat the above two steps 2 more times.

4. NOW crank the engine and see if it starts quickly. If it does, that is the smoking gun confirmation that you are bleeding off fuel pressure and it is most likely coming from the check valve.

Good luck!

Potatowalker

I'm gonna let it sit with the clamp on the fuel line some more. I've had it clamped for over an hour and it is only down to 40psi. Before I put the clamp on, it leaked down to about 20psi after an hour. I'll probably put a new pump assembly in when I get a chance. I'm going to do the "poor man's prime" for a few days in the meanwhile. This has been working for me since I got the fuel pressure gauge and started monitoring it.

I swear though, prior to getting the gauge and testing the system pressure, the "poor man's prime" would only work half the time. The other times it seemed like there was some kind of incorrect mixture problem. I would prime the pump 2 or 3 times and it would still struggle and only start if I used the accelerator pedal. I'm going to probe the living #$!! out of the TPS, etc with my multimeter if I can get it to do this again. With my luck it'll only do it again when it's 0deg F and 7am after one of my 10-hour 3rd-shifts. That's always fun trying to test a sensor when your hands are numb!

Maybe this is an example of the act of observing actually altering the situation for the better? *fingers crossed* hehehe


Would I have to replace the whole assembly with the strainer, float gauge, pump, etc. or can I just take out my assembly and replace just the pump part? It looks to be about $100 for a bosch pump and more like $300 for the whole assembly... Don't wanna buy parts I don't have to!
Anyone know the answer to this?

Potatowalker

http://www.shopjeepparts.com/genuine...ee-p-9728.html

I have a fuel pressure leak in my tank and am looking to replace the works while I have it out. Has anyone tried one of these OMIX oem units?

Any better suggestions?

40xj

not sure about the 96 but I had the exact same issue with my 98 and I just replaced the whole mod. The pump in mine was fine but the regulator was bleeding off. In these systems the regulator is stuck right on top of the mod. Try asking the parts stores if you can get just the regulator but all mine told me no. Getting just the pump wont get you the regulator or solve the pressure loss problem. The pump sits down in the bottom of the mod. then sends the fuel up to the regulator and out the line.

Potatowalker

So I've done some more investigating and have made some progress. I tested some sensors with my multimeter:

• Coolant temp. sensor tested out of normal resistance. Still had reasonable variance between hot/cold but looked corroded so I replaced that. -ran a lot smoother, but still hard to start independent of fuel pressure.
• Intake air temp sensor tested slightly off, pulled sensor and found it dirty so cleaned with carb cleaner. -runs and idles much better, but still hard to start independent of fuel pressure.

• Replaced idle air control on a whim, even after cleaning it. THIS FIXED IT! When I got the new OMIX IAC it came with instructions that said the pintle should not stick out over 28mm past the main body. The new one was perfectly at 28mm but the old one I took out was way out of spec. and sticking out way past 28mm. Put new one in, starts and idles perfectly every time as long as fuel pressure is good!

Next I still have to install the new OMIX fuel pump assembly to resolve the bleed back problem. I suspect this will resolve all my starting issues. With all the new sensors, the new CCV assemblies and valve cover seal and everything all clean it runs amazingly well compared to before. I had no idea how easy it would have been to have it run this much smoother! I think the CCV replacement and IAC replacement made the most difference, FWIW. I discovered a good symptom of the IAC starting to go bad though because my engine no longer does something that never did seem quite right. It used to do something that felt like I had downshifted a manual transmission and was using engine compression to brake, but I have an Auto. It would do this when the rpms were up and I let off the gas to coast, kinda like it should have been downshifting but wasn't. With the new IAC it totally stopped doing this! I would guess it was in fact engine compression I was feeling but only because it was starved for air, due to the failing IAC? Just found that curious, and much happier with it back to normal, smooth, and quiet(relatively speaking of course)!

CoffeeCommando

Seems like replacing the pump is the seasonal thing to do this time of year. Or maybe just this year.

I think it's weird how mine is doing the opposite of bleeding pressure, and overpressuring. Or maybe it is underpressured and the fuel level sending unit was jacked to begin with..

Potatowalker

well if your jeep is on the same rough schedule as mine, keep an eye on yer fan clutch and tranny cooler lines!