Intake Backfire

[Ben's Z]

My POS has an intake backfire. Cold, or hot, doesn't matter, happens between about 1100 and 2200 RPMS. I thought it might be the coolant temp sensor, of course not. Someone prior to me removed all the egr and cold start crap, so I have vacuum lines with bolts in the ends of the hose clamped to block everything off. A real hack job, car idles and runs like **** cold. I robbed some of the missing stuff off of ZCurves parts car but have not installed it. I slapped the fuel pressure gauge inline between the rail and the filter and I am getting about 24psi at idle. Revving the engine via the throttle shaft doesn't really change anything, it maybe goes as high as 26 to 28 PSI. Replaced fuel filter earlier this week. Some asswipe told me one time to open up a propane torch and wave it around for intake leaks, this is complete nonsense. Same guy must have made up the spraying WD40 to identify intake leaks, I just have WD40 all over the place now. Pulled thermotine wiring harness, didn't change ****. None of this is going to really matter when I put the 280zxt engine in, but I'd like to still see how weak this motor is even when running close to 100% when I pull it. I owned a Toyota Land Cruiser with an Inline 6 EFI with almost 2.5 times the mileage this engine has and everything about that engine runs circles around this piece of ****. Uses oil, slow, runs like ****.

[Zed Head]

Edit - I wrote a most excellent reply here but decided not to fuel the compost pile and erased it.

Your fuel pressure should be 36 psi. Your engine is running lean, assuming that your gauge is correct. You either have a supply blockage or a bad fuel pump. Get the fuel pressure up to 36 psi (test it with the vacuum hose to the regulator removed).

The turbo engine will be more difficult to handle than what you have currently

Edited November 30, 2011 by Zed Head

[Ben's Z]

Would have loved to know what you wrote... Anyhow are you saying that having the vacuum hose attached while getting my readings could be the reason for the low value? Or just need to check it with the vacuum hosed removed to attain the correct factory pressure readings?

Edit - I wrote a most excellent reply here but decided not to fuel the compost pile and erased it.

Your fuel pressure should be 36 psi. Your engine is running lean, assuming that your gauge is correct. You either have a supply blockage or a bad fuel pump. Get the fuel pressure up to 36 psi (test it with the vacuum hose to the regulator removed).

The turbo engine will be more difficult to handle than what you have currently

[FastWoman]

I think Zed was being diplomatic. I'd say simply, "GIGO" -- or "Garbage In, Garbage Out." Getting an engine to run well is a detailed process. Everything must be correct, particularly in an open-loop system such as these early L-Jetronics.

As Zed said, your fuel pressure should be approx 36 psi (36.3, as I recall). You need to check the fuel pressure with the vacuum hose disconnected from the fuel pressure regulator. With the hose connected, the idling fuel pressure on a healthy engine should be slightly south of 29 psi, but if your engine is running that badly, it should probably be more like 32 or 33 psi.

I would bet your lean-running issues aren't limited to inadequate fuel pressure. I suspect you have multiple vacuum leaks. You might even have a whopping intake leak from a split in the big rubber boot between the air flow meter and throttle body.

You should also check your throttle position sensor to ensure that it's working properly. Otherwise you might not benefit from either idle enrichment or WOT enrichment.

Finally, do a read on your plugs to make sure they all look the same. If they look dramatically different from each other, you may have additional issues, such as a shorted or open plug wire or a clogged/nonfunctional injector.

There are very few engines that won't run like a sewing machine, so long as they have even reasonable compression and a properly functioning valve train.

[Ben's Z]

Sarah,

What is the yogurt cup vacuum leak intake procedure again?

I think Zed was being diplomatic. I'd say simply, "GIGO" -- or "Garbage In, Garbage Out." Getting an engine to run well is a detailed process. Everything must be correct, particularly in an open-loop system such as these early L-Jetronics.

As Zed said, your fuel pressure should be approx 36 psi (36.3, as I recall). You need to check the fuel pressure with the vacuum hose disconnected from the fuel pressure regulator. With the hose connected, the idling fuel pressure on a healthy engine should be slightly south of 29 psi, but if your engine is running that badly, it should probably be more like 32 or 33 psi.

I would bet your lean-running issues aren't limited to inadequate fuel pressure. I suspect you have multiple vacuum leaks. You might even have a whopping intake leak from a split in the big rubber boot between the air flow meter and throttle body.

You should also check your throttle position sensor to ensure that it's working properly. Otherwise you might not benefit from either idle enrichment or WOT enrichment.

Finally, do a read on your plugs to make sure they all look the same. If they look dramatically different from each other, you may have additional issues, such as a shorted or open plug wire or a clogged/nonfunctional injector.

There are very few engines that won't run like a sewing machine, so long as they have even reasonable compression and a properly functioning valve train.

[FastWoman]

Yogurt cup test:

Remove the air flow meter. Stuff a yogurt cup (Yotastic brand works -- or a similar similar object, such as a cup, glass, ball, etc.) into the rubber boot to the throttle body to seal off the air. Remove the brake booster hose from the intake manifold, and attach another clean hose that you don't mind putting your mouth on. Blow through the hose to "inflate" your intake system. You should feel plenty of back pressure, and the intake should hold most of the pressure for maybe 20 sec or so. Then introduce a small leak by pulling a small hose, e.g. the one that goes to your fuel pressure regulator or HVAC control. Repeat the test, and you should feel a BIG difference in leakage rate. If you don't, then you still have a considerable leak somewhere that you haven't accounted for.

If you smoke, you might be able to find leaks by blowing cigarette smoke into the hose and seeing where it comes out. Or you might borrow a friend who smokes.

Edited November 30, 2011 by FastWoman

[Zed Head]

FW is right. Compost is the key word.

Hey, regarding the yogurt cup leak down test: I had realized in the past there is one potential difficulty with the test. Overlap. The intake and exhaust valves are both open at the same time for just a few degrees at the end of the exhaust stroke, beginning of the intake stroke. For a stock cam, according to the cam card, it looks like 30 degrees out of 720, or about 4% of the time. A 4% chance that your intake will have a big leak right through a cylinder to the exhaust. So if you get a big leak, run the test again to be sure you're not sitting on the overlap.

[olzed]

Simple solution. Bolt on a set of SUs from a 240.

Brian.

[Walter Moore]

Simple solution. Bolt on a set of SUs from a 240.

Brian.

Err... then he would have to learn the "nuances" of the SU. And vacuum leaks on a carburetor equipped car are no better than on a fuel injection system. Besides he is upgrading to a turbo motor soon so he needs the practice.

I agree that it sounds like a combination of fuel pressure and vacuum leaks. Both of which are endemic on cars once they get to be 30years old.

[olzed]

No 70s dodgy electronics to try to sort though.

[FastWoman]

Well, '70's electronics do have "character," but so do carbs. I've never gotten along with carbs. They're magic, and apparently I don't use the right incantations with them.

Zed, I had wondered about valve overlap, but I didn't have that issue when I performed the test. I suppose if a "massive" leak is felt, the engine could be turned a few degrees by hand to see whether the leak disappears.

[Ben's Z]

Going to check prefilter screen tonight, and I bought an intake/exhaust gasket to replace this weekend. Time to snap some studs off I presume.