1996 4.9 trouble shootingI am troubleshooting my 1996 4.9 straight six 300 engine. Unfortunately, I hate to say this, but since I’ve returned it from a mechanic shop, this situation has been occurring. Never had an issue with this truck for over eight months when I bought it. There seems to be an issue when it warms up 4045 minutes I get a PO10701. Which is bank one lien. I’m trying to diagnose this situation. The first suggestion is vacuum leak. That’s where I’m having a problem. Identifying the engine parts. It seems like when I look for let’s say the PV sea valve, I get one group saying it’s underneath the EGR, I got another group saying as on the backside of the valve cover, I got another group saying it’s underneath the manifold. I’m going to try to attach several photos of the engine bay and several parts of the engine that I would hope someone can identify these parts. I can’t troubleshoot this vehicle if I don’t know what I’m looking at. Thank you for any help each picture numbered so I can identify what the part is. Thank you. Rich oops, there seems too be 2 #1. Note: I have replaced all o2 sensors, cleaned egr, check fuel pump pressure.

 

Your pcv is by #8 ont he valve cover. So very back there. Remove the vacuum lines from the tree on the manifold and plug the tree. Run a new line from the tree to the egr valve with small tubing, if you reach behind there there is a bunch of tubing. Just skip the factory tubing and follow the VECI label for the egr routing. Technically the tree runs to the solenoid then the solenoid to the egr. And you can reuse the boots with new tubing. I got mine at oreillies.

the secondary air stuff is important too but less so for diagnostic. Then run the line for fuel pressure regulator

 

Your intake manifold gasket needs to be interrogated
The bolts / studs all need to be checked (they fall out)
I'd use brakleen on that gasket and see if the idle changes as you spray the intake gasket (that's a basic test for a leaking intake gasket)
Your 1996 should be OBD2 and have PIDS to check the O2 sensors switching and check the fuel trims in live data on a scan tool
Replace / check / tighten the intake bolts is first

 

goodness, so much to respond to here. I’ll take a stab at a few things. First, your PCV valve is for sure at the back of the valve cover. I too have a 96 4.9 and just had the intake manifold off. Crawl up and get on top of the engine and you’ll see the vac line going to the pcv. As for your vac lines, I found the only piece of mind I could get is to make a modest investment in a smoke machine and check it out. Those “colored” plastic vac lines are sometimes problematic, depending on how the truck has been used. Mine has been a garage queen, so they were ok. But if there are lots of miles, a hot climate, etc etc, you may have an issue. Vac lines got snaked under the intake manifold, so sometimes it’s difficult to lay eyes on them. a cheap smoke machine will tell you right away.

9 is ok, don’t worry about the nipples that aren’t connected to anything. that’s just how it came from the assemble line.
11 is not the pcv, that’s your fuel regulator.
2 and 3 are just part of the emissions crap on this truck. replacements are unobtainium.
1 is your reserve vacuum source. again, a smoke test will tell you if you have an issue there.
4 is a check valve. when I had everything apart, mine was clogged up or stuck and I could barely blow though it. the exact match can’t be found, but if you buy one for it you may have to purchase a very short galvanized pipe to connect to the emissions rail. a super easy fix, you’ll see what I’m talking about if you look at it.

I could keep going, but again, the first thing I would do is verify you don’t have a vac leak somewhere. you tube it and you’ll see how easy it is. If you get stuck and want to facetime when I’m at my truck and see a carbon copy of what you have, happy to do that.

 

Update. No progress thus far. Recapping, New, O2 sensors, checked fuel pump pressure 40 psi,(cold startup), New, vacuum solenoid, new fuel pump filter, cleaned the EGR and tested is operational. I have sprayed carburetor cleaner fluid on all of my vacuum lines no change in idle. seemed to be at the point that I’ll start the truck, run it to operating temperature, continue driving for another 30 minutes or so. I assume this is hottest point the truck can get (truck temperature is not running hot) . This is when a check engine light will throw. The truck runs flawlessly until I turn the truck off. Most of the time if I restarted immediately, it will start up fine, sometimes it doesn’t thu? If I wait about 20 minutes to start truck I get the rough idle & stumbling and misfiring. Depending how long I drive the vehicle after the check engine light comes on I get a whole slew of codes. PO 170, PO 174, PO 301 which is the EGR. I remember back in the day when I was just a kid sometimes the vehicles would get a fuel vapor lock where the gas apparently vaporizes in the fuel line? As noted above I’ve checked my fuel pressure cold so I think tomorrow I’ll run the truck till I get the check engine light, turn it off. I’ll hook up a fuel pressure gauge. Let it set for the 20 minutes and then check the fuel pressure. i’ve read a possible culprit could be the ICU unit which apparently can overheat. It’s interesting even though the OB two system you really cannot narrow down what the culprit is. And to boot to that the 1996 4.9 apparently has some unique components where others don’t. if any of you have any suggestions please share.. I’ve had a few mention tightening the intake bolts, one person mentioned to just disconnect the EGR tube and with that tube disconnected cap it off. That worked fine, but I never pursued it further because I was adding new parts along the way & wish to have the truck operate as it should. thanks for all your help

 

These issues will have umpteen opinions from various people about what it could be. Starting and then being difficult to start or running rough when it does makes me at least suspect the throttle position sensor. There are many videos on how to test them so you don’t just throw money at it. Things like this I really really dislike the parts that come from other than motor craft. ditto for nearly all “sensor” parts. Good that you checked for obvious vacuum leaks, I’ll say again the only true way to know is to smoke test, but that depends on whether your wallet can withstand an expansion of your tool set. I don’t think anyone would push you immediately towards a new pcu, but i have to admit after spending days in proper diagnosis and a never ending stream of lean codes, it turns out my pcu (ECU of you prefer) was indeed bad. Capacitors on these 30 year old trucks become an issue. I’m guessing you’ve checked to make sure your fuel pressure regulator isn’t leaking but that’s an unlikely cause if it runs fine on a cold start. on 96’2, they ae a complete pain to get to. I’m guessing you’ve popped your distributor cover off and make sure all is ok there. Unless I’m reading something wrong, it sounds like this occurs when you’re dealing with a hot engine. On my 96, while I was tracking what ended up being a bad ecu, I had nearly all the wire harness points disconnected. The crusties were everywhere. While mine has always been garaged and is clean, those connection points were terrible. It’s one of those maintenance things on a 30 year old truck I’m glad I ended up dealing with. That said, I doubt that’s the source of an issue when again it starts cold. Research your TPS, there are good you tubes on it and see if the symptoms match.

 

Randy, couple quick questions.
1 if there are crusties under distributor cap, wouldn’t it always be stumbling missing at fresh cold startup?
2 Not sure I understand using the fog/smoke machine, I understand the theory of the smoke where the smoke would go where there is a vacuum leak, but would not the motor fan compromise that whole theory because the fan would be blowing the smoke? Would you undo the universal belt? Rich

 

I count the crustiest under the distributor cap as an unlikely culprit. Reason being it runs ok sometimes. If there was an issue there, it’s an “all the time” thing I believe. As far as the smoke machine, you don’t do it while the engine is running. you disconnect the vac line going to the brake booster and hook up the output from the smoke machine there. There are lots of you tubes on it, just search for “using smoke machine” or something like that and you’ll see. No, you don’t do anything with the belt. it’s an easy test, just connect smoke and observe. You tube is your friend. Still makes me suspect the throttle position sensor, especially when it starts hard after running. Lean codes are difficult to track down. It’s easy to check the functioning of your o2 sensors, especially since the connection points are so easy to reach on these. All you need is an ohm meter. again, super easy to follow on a you tube. But the most common reason for a lean codes is a vac leak. For me, it was a long process. check o2 sensors (they were ok), check electrical connection points (they were a corroded mess), check again (lean codes remained), smoke machine (once bad connection on vac line), replaced egr valve for grins, still lean codes. in the end for me, it did end up being the ecu. it’s not the first thing anyone should point to as needing replaced, but process of elimination took me there. These trucks are old and you just need to be patient and work through one possibility at a time. Hopefully you don’t rely on it for daily transportation, if you do that’s a bigger problem for you. Too many things about these 96 4.9 trucks are unicorns. Getting the intake manifold off should be routine, but on these things because of the egr tube it’s a nightmare. Anyway, again, just take it a step at a time.

 

Thanks for the information. Just wanted to update you and I have mentioned at the top of this thread that I have new O2 sensors new fuel filter have checked the fuel pump pressure at 40 psi.
The current update is I did put in a new EGR and a new EGR sensor solenoid. Now again, let me try to be clear by running the engine considerable amount of time. Well past its normal heating range. I stopped turn the truck off and immediately started again. There’s no issue. However if I wait 15/20 minutes restart I get the stumbling and a no start. So I’m in agreement with you what you had just mentioned I’m going to buy a throttle body position sensor. i’ll keep you updated. And thank you very much for taking the time to guide me through this situation. Rich

 

what is crusties?
Show us.

If you mean debris on the rotor or the cap towers are corroded, ONLY, then it would be worse sometimes than others. if it was other stuff, it could be intermittent or constant or etc.. we need to see

 

people use the term “crusties” as a general term where electrical connections show signs of corrosion. I didn’t take photos, and of course now they are clean. it took buying some very thin tools (can easily be found on amazon) to clean both male and female ends. Was it the source of issues, I think that’s doubtful given the diagnosis of a bad ECU. But in my process of elimination I found it and saw no reason not to improve those connection points.
Rich, sorry I didn’t go back in the thread to refresh my memory on what you have done. Your issue leaves me wondering a bit because a bad throttle position sensor, in general, seems to show itself immediately on a second startup with a warm engine. Someone with more knowledge than I may say the symptom you describe (meaning waiting 15-20 minutes for the hard start to appear) points to a TPS. Wondering if you get a “gassy” smell when it’s stumbling. Also, do you have an obdII reader you’ve hooked up to look at fuel trim levels. Hate to see you continue to toss parts at it. That said, so long as I’m using good parts (never never ever ebay or many amazon), the way I look at it for a 30 year old truck it’s as much a restoration as it is repair. in fact, today I took off a rotor, shock, spring, axle pivot bushing connetion, etc. as I scored a spring seat allowing for the connection of a front stabilizer bar. us OBS guys are always futzing with something…

 

Next strategy. I am going to run the truck to the running temp. Turn it off. Wait 20 minutes. Restart. This is where I’m having the problem of not starting misfiring stumbling. So I’m thinking that we may be looking at fuel line vapor lock. So my thought is run the vehicle till operating temperature. Turn it off and wait 15 minutes. Restart & see if I get my stumbling misfire.if I do I’ll use a fuel pressure tester. Let me mention that cold start up fuel pump pressure is 42 psi. So I’m thinking I’d like to try to test it after warm-up and then let it set to get it the hottest point, I’m recapping here. Try to start it if I get the stumbling I’ll put the fuel pressure gauge on it that will tell me if there is a vapor lock of some sort.

 

UPDATE, RECAP WHAT IVE DONE
TRUCK ORIGINALLY WAS THROWING CODES 107-1074 LEAN RUNNING. ALSO GOT A CODE OF I BELIEVE WAS 401 THAT IS THE EGR.

WHAT I’VE DONE I have changed the EGR out, the vacuum EGR solenoid sensor, fuel filter, cold running fuel pump 42 pounds. I’ve run a vacuum leak test via carburetor fluid. Cleaned the distributor cap inside and rotor, they look brand new to be honest with you. There is no messy debris whatsoever. Pulled and cleaned the I believe it’s called a thin film sensor that apparently the 96 F4 .9 has that contributes to a lot of these problems.
currently
no more codes are being thrown when I use the vehicle. It starts up fine. It runs absolutely fine. It doesn’t matter how long or how far I drive it. The issue now does not throw codes anymore any time. Before or after driving. Hers the issue, the truck Wil not start within 10 minutes of sitting engine off and try to start it. Itstumbling misfiring. Above I’ve listed all the things I’ve replaced. Again no code.my thought the truck is getting hotter sitting while not running. I can’t restart after 10 minutes. I’m at a loss of what it could be other than fuel pump failure after it’s been heated up, or there’s a vapor lock in the fuel line.
next strategy
Trying to think of a way to get a fuel pump reading when it’s at its hottest point when it’s been sitting. The problem is, the Schroeder valve that I use to get the fuel pump pressure is at the back of the engine and a very tight spot tucked down into the back of the engine. It is almost impossible to get to with a hot engine. I’m not sure how to figure out how to get pressure pump connected with a hot engine. The fuel pump pressure kit I’m using is a loaner and it’s rubber & I’m afraid to connect early (when I can get to it as the engine is cool) and run the engine cause it just may melt the rubber connector hose? . Any suggestions how to get a fuel pump pressure while the engine is extremely hot? if I can get this figured out and conclude that the pressure is good, the only logical reason would be a vapor lock in the fuel line. Please pass along any ideas how to get the pressure hose connected on an extremely hot engine. Rich * forgot to mention that waiting approximately 30/40 minutes after engine cooled down, starts right up and no codes, run great.

 

ok, I’ve thought on this. Your quick summary is your issue is related to heat. what is my short list? I don’t think it’s fuel related, as unless you consistently run with 1/8 tank of gas the fuel pumps typically don’t work fine, then stop related to heat. Since heat is the issue, I’d go with what may be heat related. For me, that narrows it down to an ignition coil (fairly cheap to replace), an ignition control module (these can be impacted by heat), and unfortunately I have to also include the distributor pickup coil. That coil is an involved process to replace, but I would not be surprised if that was it. You clearly have spark, you have fuel, the truck runs fine, It’s heat. Those are my top 3. I’ll defer to brighter minds than mine, but if it is fuel pressure related to heat, I’ll look forward to being educated on it.

 

Scan tool testing and vacuum leak testing
A 1996 4.9 with a DPFE sensor (I see in the picture) means it is OBD2
So, I'd start with a OBD2 scan tool looking at fuel trims first, for the lean code
Same with DPFE testing for the code P0401, it's pretty easy to do those pinpoint tests for code 401 in the PCED to determine if you need a new dpfe sensor
Pretty rough getting a new DPFE that will not turn on the nlight with one of the old aluminum ones
Every plastic one I get, they say works, doesn't