backfiring with flame through throttle body

[highlander]

Hello to all
Any help, would be greatly appreciated, let me start by starting at the beginning. Recently I ran across someone who wanted me to help fix their 93 Buick roadmaster v8 5.7 . he explained to me that it is been experiencing a very rough idle, they stated that the fuel pump had been replaced, they had also had the spark plugs replaced. And they believed that something was wrong with the throttle body injector.
So I told him I would take a look at it, so I began by attempting to start the car, I did witness a sporadic fuel spray pattern from one of the TBI injectors and I ordered the rebuild kit for it. I then went on to inspect the distributor cap and rotor, the distributor Was slightly worn and the rotor had a crack in it, needless to say I replaced them. I checked the plug wires all were within ohms specification, that ohms reading I got was as follows starting with the shortest wire going to the longest.
　
1: 4,13 k ohms
2: 4,13 k ohms
3: 5,18 k ohms
4: 5,55 k ohms
5: 5,55 kohms
6: 5,98 kohms
7: 6,17 k ohms
7: 7.04 k ohms
Which according to the manual all is with an specification, I also took and tested the ignition coil and the ignition module both tested good, but I double checked and had AutoZone re-check them. AutoZone said: they were good.
So the fuel pump has been replaced, the throttle body with the throttle body injectors and pressure regulator, and fuel pump, and spark plugs have all been replaced.
The ignition coil, plug wires, and ignition module has all been tested. I have checked for vacuum leaks and have found none, well except for a little one in the Egr valve diaphragm, which I tested using a handheld vacuum pump, but I was told by a local garage that a leaky Egr valve. Would not cause the backfire with flame coming out of the throttle body, is that true? that
I should check the timing/compression by aligning the timing mark on the crank pulley with the corresponding timing mark on the timing degrees gauge located above the pulley and check to see if the distributor rotor is lined up with the number one firing cylinder cylinder. Which I have checked the timing according to the manual the timing must be set at 0 btc, so I aligned the marks checked the rotor under the distributor and it was not pointed to the number one firing cylinder. It was slightly past that point. So I loosened the hold down bolt, and turned the distributor by hand until it was aligned, I then re-tighten the bolt. I started turning the engine over by hand and noticed that I was able to turn the engine from 0 BTC to about 4 in either directions 4 after or 4 before the 0 , without the distributor rotor moving, is that enough play to cause a backfire and for a flame to exit through the throttle body?
So I guess my questions are what should the resistance reading be on the throttle body injectors? Can a leaky Egr valve diaphragm cause backfire. And is the slack in the timing chain enough to cause backfiring. Oh I did start testing the compression but have not completed it yet, but I did complete all four cylinders on the right passenger side the readings of the compression was
1: 110 psi
2: 140 psi
3: 105 psi
4: 130 psi
　
All of them are above 100 but there is a couple low ones do have me a little concerned.

Oh, and I did put it on a code scanner and no faults where present.

　
Any advice is welcome.

Topic

[Joeseph Mama]

it is been experiencing a very rough idle,

backfire with flame coming out of the throttle body

bad timing.

Setting timing for each specific engine is different; you must refer to the appropriate procedure for setting ‘base timing’ for that particular engine. There may be other factors at work here, such as a loose and/or skipped timing chain which you would be eligible for with an older high mileage vehicle.

A chilton or haynes manual would be good here.

[Bill]

First thing I would do is check the firing order clockwise from # 1 cylinder at TDC. 18436572. After all the hands that have been in there it’s possible some plug wires got crossed.

It sounds like there is some slack in the timing chain but unless it’s jumped time it should run.

Some GM EGR valves won’t open with vacuum applied as they require exhaust backpressure to operate.

[highlander]

Thanks to all for the info

to keeps things straight I’ll try to address each one individually, and with as much detail as I can.

Ratchet Face……. actually, that’s what first popped into my head was bad timing, some sort of ignition problem. And I tested the ignition modular and the plug wire and the ignition coil, then replaced the distributer cap and rotor, also replaced the plugs.Which pretty much covered the ignition possibility. Or at least I believe it covers most of. I then started looking at timing itself, possibly a slip timing chain or slack in the timing chain.

I do like Haynes manuals but here recently I’ve been using alldata, this is where this bit of information came from, and this is what I used for setting the base timing

a. Remove the number 1 spark plug.
b. Place finger over the number 1 spark plug hole and crank engine slowly until
compression is felt.
c. Align timing mark on pulley to “0″ on engine timing indicator.
d. Turn rotor to point to number 1 or number 8 spark plug Towers on the distributor.

e. Install the distributor cap, and tight hold down clamp.

And that’s exactly what I followed in order to set the baseline timing. It was after I had completed those steps that I realized I was able to turn the engine by hand from 0 to 4 above or 4 below, and the distributor rotor did not move. And the backfire was still present. I did run a smoke test to check for vacuum leak and I only found one in the EGR valve. But I was not sure if that would actually cause a backfire with flame present in the throttle body upon the backfire. Or really what all could cause the type of backfire. I just kept wanting to go back to ignition as being the most likely cause. That’s when I decided to run compression and I got some unusually low numbers I did complete the compression test both wet and dry (by adding oil) the W means when oil was added.

1: 110 psi
W:145 psi

2: 140 psi
W:150 psi

3: 105 psi
W:150 psi

4: 130 psi
W:155 psi

5: 135 psi
W: 155 psi

6: 130 psi
W: 150 psi

7: 115 psi
W: 135 psi

8: 100 psi
W: 120 psi

Thank you

Ratchet Face for the info and reply.

[highlander]

[quote=”wysetech” post=110528]First thing I would do is check the firing order clockwise from # 1 cylinder at TDC. 18436572. After all the hands that have been in there it’s possible some plug wires got crossed.

It sounds like there is some slack in the timing chain but unless it’s jumped time it should run.

Some GM EGR valves won’t open with vacuum applied as they require exhaust backpressure to operate.[/quote]

You know, I did think of that, our heads seem to be in the same place at that point. So unless I’m reading the alldata wrong this is actually how I connected the plug wires with the distributor. I uploaded a picture with color lines in a colored dot under each of the firing order to indicate where he ran plug wire from distributor to spark plug. If you don’t mind and not too much trouble you can take a look at the photo and tell me if I got it correct.

And you bring up a great point. About the Egr valve, you would know how to go about testing a exhaust pressure one… Would you? I can only think of applying air pressure as a means of testing. But I’m still not sure if a leaky EGR would be enough to cause that type of backfire condition. I did talk to a couple of garages and got conflicting information. As one said; that a Egr valve could not cause that type of backfire, and that the compression even though one cylinder is down to 100 psi they could go down to 90 and still run fine, to stick with ignition/timing as the cause. Now the other garage agreed that the Egr valve would not cost that, but the low compression indicated more of the worn intake valve which could cause that type of backfire, and that anything below a 100 psi would give problems (alldatta also states this). And since I had adjusted the baseline timing according to alldata… it should of started. But they believe it’s a leaky valve and that the spark is escaping from the cylinder and igniting the fuel in the intake which leads to the backfire out of the throttle body, but that even if I did take care of the slack in the chain, assuming that it the slack is causing a problem, that low compression is a bigger one, and that it is the compression that is the real problem.

I however will recheck the position of the timing marks on the pulleys and timing indicator, I will also check to make sure the plug wires are ran correctly, so if my color lines are leading wrong please by all means pointed it out.

Thank you Wysetech for the replies and info.

Oh one last little question if you don’t mind, in the picture of the firing order it shows a distributor At the top right to remote lines run the distributor To each cylinder. It shows a small white circle at the bottom of that distributor, and a black small circle at the top am I correct in assuming those small circles represent the screws that hold down the distributor cap?

[Ingvar]

This is your answer:

I started turning the engine over by hand and noticed that I was able to turn the engine from 0 BTC to about 4 in either directions 4 after or 4 before the 0 , without the distributor rotor moving,

Pull distributor out and check on worm gear. If it’s worn, or loose on the shaft, as that what it sounds like, you have your answer. Of course dizzy will cause misfire.

[Bill]

Yeah, your firing order appears correct. A bad EGR valve will not prevent it from running.

I’ll keep scratching my head for you.

[Author]

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