1990 302 EFI Erratic/Low Idle & Engine Knock
 

I've had my 1990 Bronco Custom for about 6 years now. It had just over 87,000 when I bought it. It now has over 112,000. I'd say in the the past 5,000 miles or so, it has devloped an erratic idle(accompanied by knocking sounds coming from the lower end). It only happens right after start up on cold mornings(like this morning), especially if I use the Heater. It will "try" to idle for about 10 seconds or so, then die, resulting in me having to restart the vehicle. Otherwise, after I have driven for about 20 miles(my daily commute to work), the knocking stops, but the engine idles rather low. Any suggestions, folks? :-X16

Yes, do a tuneup and have a close look at the oil level. Check for codes. Has the timing chain been changed ever? i think it could be time to replace it.

Oil Levels are good the last time I checked(yesterday afternoon). And to my knowledge the Timing Chain has never been replaced. I was thinking the knock might be a Main Bearing, or(hopefully)just a loose Rocker Arm that needs re-tightening. And yes, I'd say a tune-up is long overdue for my truck.

Could the knock be an exhaust leak? Im pretty sure I've got one going on and it goes away or at least quiets down a little after engine is warm. For the idle I would clean the IAC valve if you haven't done so already. I had an idle problem a few years ago and cleaning the IAC smoothed it right out. Good Luck

I have a 1990 too, purchased in 2001 with 87k and now has close to 160k and runs like a fine Swiss watch. I once had similar problems, and it was an exhaust leak. In my case, it was the passenger side exhaust manifold. One of the bolts broke off by itself, the rearmost one, so there was an exhaust leak there. When I first heard the exhaust leak, the engine would also ping, but that pinging eventually stopped, I guess because the computer reset itself after a while. Depending on where an exhaust leak is, it can mess with your computer and cause various problems. But the thing that most makes me say that it's an exhaust leak is how it is louder when the truck is cold. That's what exhaust leaks do, never figured out why but they are louder when cold. This is assuming you have the proper amount of oil in your engine and no mechanical problems such as a bad main bearing. As for the erratic idle, there are many things that can cause this, including an exhaust leak. The IAC is usually the problem and it's super easy to replace. You can clean it out but why bother, I just got a new one. They're not expensive. Also, the EGR valve riser can become clogged, and that will definitely screw with your idle. My idle was very erratic until that tube was removed and cleaned. Now my idle is so smooth that you can't even feel the motor running and sometimes I have to check the tach to see if it's still going! But first I'd check your exhaust system closely from the manifolds and manifold gaskets and bolts to the last sensor and the cat and make sure there are no cracks in the manifold or broken off bolts or holes in pipes before sensors. If there is an exhaust manifold leak it's mostly likely to be at the rearmost cylinder on the passenger side, but not necessarily. Anything after the last sensor and cat shouldn't mess with the truck's operation, it would just be louder. There's also an electronic control module mounted on the passenger side of the firewall. If it's bad it will cause an erratic idle but it would not make knocking noises. It's just two screws and can be changed in five minutes. After checking the exhaust system, IAC, EGR tube etc. if you find nothing wrong go to an auto parts store and have them scan the codes, or do it yourself. But it can be tricky on a 1990 as you have to remember sequences of beeps and lights and then look them up in a book. I have a code reader but I stopped using it because I was always forgetting the sequences, even if I was writing them down I'd always get mixed up somewhere. Finally, don't rule out the fact that there may be more than one problem. You might have an exhaust leak as well as some sort of sensor issue. Hope this helps!

I third the exhaust leak on the knocking sound.. mine at the manifold always sound worse on a cold start. I know some oils cause a knocking sound for me on cold days with cold startups.. but my motor has twice as many miles as yours..
You also just might be hearing some of the water that accumulates when cold being burned off by the motor. Does your truck blow a ton of steam out of the exhaust when you first start up in the mornings?
Scary enough, that's all coming from the oil and it's perfectly normal. If you don't see it, make sure your PCV valve hasn't seized!

As for the low idle and dying, could be anything. Is defrost on with the heater? that will also turn on your AC compressor which puts a bigger load on the engine and will drop the idle. If it's a bit out of time, has a dirty IAC or something along those lines it could definitely push it to the point where it dies, especially on cold mornings.
It might also be weak fuel pressure, slow injectors, clogged / frozen PCV valve, all of which are common on these trucks in the cold if things haven't been changes.

did you check codes on it? Do that first.. it might point you to exactly what's causing it. TPS or IAC or .. well, there's plenty of sensors that could cause this. If your EGR valve is stuck open for example? just run codes

yep, id check for an exhaust leak aswell, my 93 has a awful leak at the manifold and when u start it, it has a hesitation and seems like it wants to die, but eventually idles at normal speed and the "ping" noise i have goes away at higher RPMS then lower or idle

what oil are you running and what oil filter?

Right now Im running some el-cheap-o Wal-Mart brand oil and a Napa Gold:-X0A6 filter. Normally I run Chevron Delo 400LE oil though...

EDIT: I decided to check my Radiator fluid for the hell of it when I got home not too long ago, when I noticed a blackish/brownish sludge oozing from the Breather Valve(that connects to the drivers side Valve Cover). Now I wonder if I have a Head Gasket going out...

Excessive oil coming from the valve cover vent usually means worn or cracked piston rings.

Check your oil with your dipstick and make sure it's not full of water there, it will usually settle above your oil.

In this cold weather it takes 3 minutes of running before I get all of the water out of my oil after it cooled off all night. You can see the top of my passenger side valve cover, it's full white and it just shoots water through my PCV valve into my intake.. but after warmup it goes away and it's not burning coolant, it's just normal condensation.

At low throttle you should get very little venting out of your breather (driver's side) if your PCV is operating and your motor isn't blown, if you unhook your PCV valve you should see a lot of venting out of your breather. At full throttle you should get some visible venting even with the PCV hooked up, it will look excessive when cold, always, however it should rarely spit oil.

It's normal to collect SOME oil in your breather hose with normal driving even with a healthy motor though. The main point of the PCV is to reduce gasses from your motor running, most of those gasses are created from water that collects inside your motor during normal operations and some oil is going to creep out of your breather, especially the oil that emulsifies with the collected water, it's unavoidable.
Usually you want the stuff coming out of your PCV to be closer to white than brown (more water, less oil) but the closer to the valve cover the browner it will be.

Make sure your breather is sorta far away from your intake to reduce the amount of oil you burn. I tried running mine pretty close and it was sucking up oil like mad and affecting the way the truck was driving. If you have it too close to your throttle body, there will be too much suction and it will pull out more oil than it should.

It's usually when people see puddles of oil in their intake or it starts squirting out of the breather than people think the rings are really worn.

As for the Breather, I bought an extended hose for it awile back. So now the little Filter I have attached to the end of it sits on the Fan Shroud between the Radiator Hose and Radiator. I moved it there 'cause the gasses it produces nearly choke me out inside the cab unless I have the heat(or AC)running, or I have the window down. Do you suppose there is a seal of some type in the Firewall/Dashboard that needs replacing?

You shouldn't use a breather filter, you need to run it into your air box so the fumes get sucked into your intake.

I have a custom intake, but I think the stock one has a spot for you to run it to? you really don't want it just filtered, I dunno why people do that.. EFI's are built to take this into account, for example the high idle at first start up also helps aid burning off of condensation, it isn't just for the faster warm-up. Though honestly, the breather running to your intake is just to keep your compartment clean and the fumes out, there isn't enough vacuum to matter

You want full vacuum to your PCV valve (Motorcraft only! the others do NOT work! this is one of the most critical parts in your engine) and throttle vacuum to your vent tube, but make sure it's a couple of feet away from your throttle body, otherwise you'll suck up oil.
Any other setup is flat out wrong.. though it's acceptable to use an oil catch can for your breather only, not for your PCV, and that's only if you run a lot at high RPMs, have a really worn motor or use forced induction (supercharger/turbo), or if you have to run your breather close to your throttle body because of space issues.

Also, it always helps to make sure you do NOT shut your truck off if it doesn't reach operating temps. At 192f the water will all but evaporate and be sucked in through your PCV valve ad into your intake, any cooler and it will emulsify with your oil and cause your oil to thicken and sludge up. Usually you're safe at a lower coolant temp, operating temps is just a good safe point. This isn't always possible, of course, just make sure it's warmed up as often as you can, otherwise water stays in your oil, emulsifies (mixes) and doesn't get removed as easily (can get stuck in bearings, sludge up and get sticky, thicken your oil and cause mechanical issues, etc)
That's a big reason for why factory thermostats don't go much lower than 192f.

Sorry, I just learned all of this stuff within the past few months and I think it's pretty cool. I use to think I had a leaky head gasket before I found out all of this info.

and your engine compartment should be clean of all fumes.. it took me a year but I finally got to that point, so it is possible. If it's stinking in your cab make sure your exhaust is sealed, the exhaust manifolds in all of these engines loosen over time.. seriously, all of them do, it's only some bolts as well, some wanna fall out and some stick to the point they have to be drilled out.
The fumes makes me think exhaust again with your knocking engine.

there is also that EGR tube leading from your passenger manifold to your EGR valve ,those have a tendency to break without you being able to see the break and they can stink up your engine compartment pretty good and make a knocking sound.

Then of course, leaks.. oil at your valve covers is the worst because it leaks down to your manifolds, burns off and gets sucked into your blower/heatecore box and into your cab, or in any firewall openings.


I do a smell check every few times I get out of my Bronco.. if I smell anything under the hood, I know something needs fixed or tightened!

get the delo back in there and like mentioned get that breather tube run back into the airbox.