Help,

I have a 90 F150 with the 4.9L and standard transmission.

The truck has never seemed to run as good as other 4.9L I have owned. The trouble however has realy gotten worse over the last couple of months and now the truck is virtually non-drivable.

The TFI modle went bad about a month ago, I did not feel like messing with the truck at the time ( had to many other things to do that week) cost me $400.-- for the dealer to replace the part. Got the truck back, truck runs but not very well. One lessen learned. Do that job yourself. No matter how mad you are that day at your truck

So I checked the timing everything is ok.

Bought a $30.00 code reader. no codes.
expect for everything fine.

Checked fuel pressure 58 psi key on, 48 truck running.

Replaced spark plugs, fuel filter, distributer cap, plug wires during normal maintenance 4 months ago.


I guess I can explain now what the problem is.

The truck runs extremly rough at idle and is at the verge of almost dying . Has very little power at low RPM, and problem seems to smooth out as RPM raises abouve 2500 (I dont have a tachometer so that is a guess) or least not get any worse.

A couple of questions:

1. Would clogged exhaust cause this, even though the problem does not seem as bad at higher RPM. Which I would think plugged exhaust would make the problem worse at higher speeds.

2. Why would I not get any trouble codes. I would have thought the computer would have picked up on the truck running rough.

Any help would be greatly appreciated.

 

I believe that misfire detection was introduced with OBD2 in 1996 . Our EECIV setups don't have this capability so that would explain why no codes are set.

You may want to start by checking wire routing for possible crossfiring considering you already performed maintenance in t hat area. Then remove the plugs and note their condition and colour. Also note if any plugs are running leaner/richer than the others to determine if the misfire is confined to one cylinder.


An exhaust gas test will probably reveal high HC levels, along with a sooty coating on the tailpipe.

 

Problem with computers is that very strict conditions must be met before they will set a code.

This could go any number of diff ways checking plug condition will be a good start. This could be a faulty coil but further diagnostics is needed. These kind of problems are sometimes the hardest to find.
Think I'll just watch this as it progresses and maybe get an idea. (inspiration) Sorry I can't be more right now.

 

Might possibly be EGR related, but thats pretty hefty.

Did you try running injector cleaner through it?

 

Thanks for all the suggestions.

I am guessing that Haynes manual will have proper routing of plug wires? I will check later tonight and see if my manual has wiring routing. I will also check the plugs.


I forgot, I relplaced the coil after I got the truck back from the dealer. But before I tried any systematic approaches to fixing the problem. So chaulk that one up to me not testing items.

Rhetor, I did run some fuel addivitve I think it was Chem Tool or were you talking about cleaning the EGR?

I have a couple of days off, so Monday morning I will check all you guys suggestions. I will let you know what I find.

 

Yes, fuel additive is what I meant. Sorry if that was unclear at all.

Although a malfunctioning EGR shouldn't give you problems as severe, it may be making a small problem worse.

I would try disconnecting the green vacuum line that goes to the EGR. If this makes the problem better, try making a restrictor plate.

If that just helps and doesn't make the problem go away, and you do find the source of the problem, you should reconnect the EGR or remove the plate.

Hope you have some luck.

-Tyler

 

Well I checked a few more things today.

First I checked the spark plugs, below are the conditions of the plugs.

1. Shiny brown glazing on half of white base.
2. Clean very very clean. There was a slight brown color that I almost did not even notice.
3. Same as plug 1
4. Same as plug 1
5. Light grey soot.
6. Very light brown, not a glaze looke like plup 1.

When reinstalling plugs I checked the wire routing.

Tested truck, problem still there.

Then I decided to pull each plug wire off one at a time to see if problem changed. Problem did change with each cylinder. So my guess all cylinders are getting fire.

During this test I did come to a better discription of the problem my truck is having. This may help some people give me some ideas. When I pulled a wire off of a cylinder the truck and ran the truck it would studder and was slow to increase RPM. I would say that my trucks problem is extremly similar to this condition but is about 1/2 as bad. A better way to explain this may be to say that taking a plug wire off the egine increase the problem by 2.

Before I could do much more testing it started raining My next plan is to check the fuel injectors one at a time by unplugging them. I have already noticed that this looks extremly difficult to get to each of the injectors. If someone has another idea let me know.
All help is appreciated.

 

Almost forgot,

I also checked the EGR valve by disconecting the green line, no change in problem was noticed.

 

Pulling you injectors will do the same thing as pulling the plug wires. I would suggest doing a compression test and if they are all pretty even than my guess would be the #2 injector is weak.

 

Plug number 2 looks very suspicious. Perhaps that injector is partially clogged.

I havn't done a cylinder balance test in a long time but you can do a poor mans test by hooking up a tach and unplugging each ignition wire in turn and noting the RPM drop. A small drop indicates a weak cylinder. The correct way to perform this test is to unplug each injector and not the ignition wires but as you said, these are very difficult to get to. You also have to unplug the IAC or spout ( maybe both, I forget) when performing this test.

You can also listen to the injectors with a mechanics stethoscope or a long metal rod of some kind. If an injector is partially clogged then it may ( not always) sound muffled compared to the rest.

 

baylesr:

An easy check (IMHO) on whether you're getting crossfiring due to misrouted plug wires is the water test. Take a clean spray bottle of water, and a flashlight, and go out when it's pitch black, and, after orienting yourself away from the movin' parts, spraying the ignition wires, cap and coil. If there's any light leakage, you've got poor wires, regardless of whether they're "crossfiring" or not.

It helps to have an assitant inside, to rev it as this changes the intensity of the leaks, making them more visible, if you've got them. Look closely, as leaks can be somewhat dim and hard to see--that's why it's got to be pitch black out to do this.

Bigbad: I will try to listen to my injectors with my mechanic's stethascope, re: my "Help. Fuel Delivery Problem?" post, a few pages back, as it is once again running crappily, but not as crappily. Started, but still backfiring and poor power. Since my idle is smooth, I assume listening to the injectors won't tell me anything, rigth?

Thanks.

Peter

 

A symptoms of a bad injector would be most noticable at idle so if yours is idling good then your problem probaly lies elsewhere. You may want to bump that post considering the problem returned.

 

Bigbad:

Thank you, re: the injectors idling okay probably = not an injector problem. The truck idles well for 170,000 and better than a lot of vehicles, regardless of mileage. I think it used to idle smoother a year ago, but still, the gearshift wiggles only slightly as it idles.

I will "bump that post" (like that) tomorrow, but I wanted to cut my old fuel filter in half, and do anything else I can think of, so I have something exciting to talk about.

Oddly, I drove it again tonight and now the backfiring has perhaps completely disappeared, or is down to a burble normal for only having a cat and no muffler, but the power is still down some. Weird.

Thanks again,

Peter

 

Just to update in case anybody is reading this and having the same problems.

The problem is still there, and here is the list of things that were check or relplaced.

Timing Checked- ok
Fuel Filter- replaced
Spark Plugs motorcraft- replaced
Spark Plug wires motorcraft- replaced
Distributer cap checked- ok
Fuel injectors- flow tested ok.
IAC checked- ok
TPS checked - ok
EGR valve cleaned and checked- ok
Fuel pressure checked- ok
Vacum checked idling and driving- ok
Coil replaced (old one was ok though, I think)

Have not done a leak done test yet, that is on the list.

Spark appears orange not blue.

Just wanted to post the information in case any one else is having the same problem. Rough idle (feels like choke is stuck on a carberator even though the engine is FI) and studders under accerleration.

 

Do you have your backfiring when you are on the gas or when you let off? Starting to sound like a bad valve. This should show up in a compression or leak-down test. If your having it when off the gas though it's probably just an exhaust leak.