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1991 TBI; verifying timing advance with RPM
#781694 September 04th 2021 10:45 am
Joined: Feb 2018
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Hi all,

My 91 B250 with 318 TBI broke down last month on a weekend trip into Eastern WA. Almost everything cut out and it died, would sporadically not restart and would instantly die in gear. We'd been driving for a few hours in 95-100 degree temps and lots of wildfire smoke, but all was good until it wasn't in terms of power, temp, voltage, etc.

Anyway, got the van back home to Seattle and was doing some diagnosis last weekend. Turns out initially I had some corrosion and wiring issues. After redoing all the battery to engine and engine to frame grounds, and using a crap ton of cleaner and new dielectric grease on basically every connection under the hood and at the engine. I can now get it to start most of the time, and once running it will idle all day in the driveway, and not die in gear, but there is a bad hesitation when trying to accelerate.

I've gone through with the manual and tested resistance readings on all my throttle body sensors (ended up replacing the TPS with a new one it was out of range), and verified 5v signals to and from ECU. Also verified 14psi fuel pressure at idle and when trying to accelerate by hooking a pressure tester up at the throttle body and driving around my block with the engine bucking and shuttering, my neighbors were highly amused, haha! I also installed two new injectors, which was what gave the engine ability to actually idle in gear. So I'm fairly confidant about all those things being OK.

Last Sunday I had the idea I wanted to verify that the computer was advancing timing with RPM. So got the wife and my timing light, verified base timing (with coolant temp sensor off, 10*), and that with the sensor plugged in that timing at idle advanced to about 20*. The issue I see is, that when I had Aly rev the engine up to 3000 RPM in neutral I did not see timing advance past that 20* at all! I would have expected it to climb into the 30-36* range. Keep in mind this is not a mechanical advance dizzy, it uses signal from the hall effect sensor to tell the computer what the RPM is. And the computer is then supposed to modify appropriately.
My FSM only has one section about verifying timing advance, and it talks about using a plug in scanner tool to tell the engine to run a test that advances the engine to X RPM and to see what the result is. Not helpful since I don't own that tool ;-). But, my gut is telling me, that since I cannot see timing advance with my light, that the ECU might not be advancing timing with RPM correctly. Which would explain my hesitation.

Curious if anyone here can verify my theory or have any other helpful idea for things to check? I'm hesitant to drop $280 from Rockauto or O'Reilly
s on a re-manufactured ECU without a sure diagnosis. I've also been looking at some of the independent rebuild companies that are on EBAY, because $150 is better than $280, there is just more time involved with needing to mail my ECU to them and wait for a repair and return.

Cheers all,

- Rich


1991 Dodge B250 Zephyr Conversion. "Bi-frost" 74,000-ish original miles and counting.
- 318TBI; has headwork and other bolt on upgrades. Runs strong and smooth!
- Powertrax locker
- 3" aluminum radiator.
- Rear air springs, rebuilt suspension, steering stabilizer.
Re: 1991 TBI; verifying timing advance with RPM
RichardB #781708 September 05th 2021 5:29 am
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several beers deep.

Have an 89 b250 TBI, 20 plus years now.

Hesitating on accelleration, falling flat, spark plug wires.

There was TSB on some of this era, regarding coil wire location in regards to adjacent spark plug wires. I believe it was with a mroe rectangular coil monted at front fo engine vs cylindrical coil next to distributor.

My factory service manual says the coolant temp sensor has the biggest effect on spark timing, IIRC.

DO you have any computer codes.

Bucking and surging will likely cause a code 13, map sensor code.

Have you cleaned the throttle body? those vaccuum port sources towards front of engine, get clogged. Have some gasket maker on hand as the gaskets rocksuto sells might or might not line up with the TB.

My 89's ecm has a 14 pin connector, and a 60.

The failure of the 14 pin connector has been a source of stalling, bucking, burping/ farting/ surging on mine, in the past. The weight of the wire bundle breaks the solder joints where pins meet circuit board. Zip ties excerting pressure on connector/ecm can copmpletely mitigate the issue, for some undetermined period of time. Years in my case, but failure was stressful, but also inspired learning how to solder properly, a skill I enjoy and wish I developed sooner.

I believe the 91 ECM only has the 60 pin connector. Perhaps when it is acting up and wont start, see if keeping the key to on, not start and wiggling that connector, has relays click on and off.

Another ecm diagnosis which might work, if/ when it refuses to start, see if heating it with a hair drier changes that.

regular electronic contact cleaner, like crc qed, is awesome at blasting old dielectric grease from connectors, but it does nothing for resistance inducing oxidation on those connectors. Granted the di electric grease should have prevented that from occurring, but were talkng close to 30 years assuming the dielectric grease was used correctly at factory floor and never manipulated nby techs in the subesequent timespan.

Caig Deoxit d5, or d100 on the really heavily oxidized connectors, can make huge differences in contact resistance. I pulled ancient di electric grease from my original connectors, which looked like somebody taped both mating connectors with scotch tapem then used some channel locks to mate the connectors.

number 2 cylinder spark plug wire on mine, has been a cause of hesitation, and a brief misfire under load ,more than once. My DMM revealed I was able to change resistance of that wire significantly, by moving one specific part of it, kind of where mid way between cylinder 2 and 8 resides when installed.

Keep your spark plug wires from touching each other, or any grounded metal.
Keep in mind they often get pressed down and in when re installing the engine cover.

Also bucking and surging will cause the wire bundle from valley to exert different pressure on the wire bundle into engine computer as the engine gets torqued, and leans over on its mounts.

The location of the ECm directly atop the engine on firewall is also asinine. Excessive heat reduces the lifespans of electrical components, and the heatsoak on engine shutdown, and then cold restarts 12+ hours later can pt lots of stress on connectors to from ECM and cause weird head scratching, tool throwing, curse fests no matter how much one identifies with sherlock's power of observation and deduction and tries their best to employ such tactics.

The fully functional back up ECM, will make one heck of a diagnostic tool.
I intend to own one soon.

Re: 1991 TBI; verifying timing advance with RPM
RichardB #781724 September 05th 2021 4:12 pm
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Im glad you've already tackled the wiring harness... that's always the big gorilla in the Dodge room..

Have you taken a look at the transmission and shift selector linkage and any kickdown adjustments? If they get out of whack, the symptoms can range from early or late shifts to genuinely alarming bucking bronco jolts. The FSM details the procedure for adjustment.

Make certain that none of the Nylon pivot bushings at the ends of linkage shafts have popped out of position. If it happens to be the shifter linkage, you'll possibly find yourself dropping in and out of gear.


-It's been such a LONG TIME... BlueShift>> 1981 Dodge Ram B250 Custom Sportsman Maxi Van


[Linked Image]

It's what you learn after you know it all, that counts...

Are you living to work, or working to live?

[Linked Image]

Learning from my own mistakes is good, learning from yours would be much better! [Linked Image]
Re: 1991 TBI; verifying timing advance with RPM
RichardB #781727 September 05th 2021 4:51 pm
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I don't speak dodge, but make sure you can even get full advance in neutral or park. It's not uncommon on the Chevy's to lock out advance thru first gear for emissions. When GM started installing TBI, they repurposed the reverse-safety neutral switch to a reverse- in gear switch to tell the ECM to turn everything on. When I finished my manual conversion I had to put a switch in to tell the computer the van is moving.(still running on the Auto ECU)

Re: 1991 TBI; verifying timing advance with RPM
RichardB #781923 September 08th 2021 6:11 pm
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My money would be on a ground somewhere, even if you sharpened them all up there might be a bad wire. I wish I could help hands on. When I had mystery issues with my TBI engine it turned out to be ONE of the injectors, the one that was hard to see when leaning in over the drivers seat. The 5th time I checked them I happened to be inside the van between the seats and looking down on them BOTH (finally) and suddenly there was the problem.

I have an ECU I got from a junked B250 of around '91 vintage. Because it turned out it was not the right one for my van I was planning on putting it on eBay. If it is right for you then maybe we can give that a try? PM me if you want to go down that route, I only want $50 or so to claw back some of the money I wasted by not checking my ECU before I went to the junk yard


1988 B250 Conversion Van
https://i.imgur.com/uQPPuqo.jpg
Re: 1991 TBI; verifying timing advance with RPM
RichardB #782003 September 10th 2021 12:52 pm
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Hi all,

Thanks for the suggestions. I haven't had time to dig back into the van yet. But think I will be able to on Sunday afternoon. I did actually score another ECU from a junkyard van this week. So that will be a quick and easy swap and test. I also picked up a bunch of new vacuum hosing and am planning on going through and replacing all of that. Back in the mid-late 2000's when I was in college I would buy 1980's Toyotas that "didn't run right" for cheap and flip them, which usually meant just spending an afternoon replacing all the vacuum lines one by one and cleaning out the carbs.

I did not check the vacuum port for the MAP sensor, I just did the vacuum pump test on the MAP that's in the manual, and that checked out. But I do own an ultrasonic cleaner, so I may just pull that throttle body and tank it. Easy to do, and the gasket kit is only $18 after all.

I also love the check transmission linkage idea. Specifically the kick down cable if a little bit off can cause grief.

Will report back once I figure it out. Cheers!

- Rich


1991 Dodge B250 Zephyr Conversion. "Bi-frost" 74,000-ish original miles and counting.
- 318TBI; has headwork and other bolt on upgrades. Runs strong and smooth!
- Powertrax locker
- 3" aluminum radiator.
- Rear air springs, rebuilt suspension, steering stabilizer.
Re: 1991 TBI; verifying timing advance with RPM
RichardB #783573 October 10th 2021 10:10 am
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So I've only been able to play around with this thing on a couple afternoons in the last month. I have:

- Sourced 2x junkyard ECU's. Both work, van will start and run on them both (more on that below), and on the old computer I've had for a couple years. It will also start and run on the original computer I have, which has a bad voltage regulator circuit (no alternator charge). So at this point given how much plug cleaning and 5v signal testing I have done I am ruling out the ECU.
Also as an FYI, not surprising, but the computers from the same vintage pickup trucks are the same. And they are mounted in a nice cool spot up front in the engine bay on the drivers side fender. Seems much nicer than the right over the hot engine van location.

- Replaced all the engine vacuum hoses. Pulled and cleaned out the throttle body vacuum ports. Sprayed starting fluid all over to verify no vacuum leaks This made no appreciable performance difference, but it looks nicer and was cheap/easy to do.

- Yesterday I thought I hit a breakthrough. I started removing spark plugs so that I could compression test the engine. One of the spark plugs broke in half when I was removing it. I set it down on the floorboard and the tip just fell off. "Huh! That's not good!" I said. So a new set of plugs, wires and dizzy cap and rotor went in. My van fired right up, perfect idle, no in gear sputter. So I proceeded to start driving around my neighborhood victoriously! cool That is at least until the engine got up to full temp, and it suddenly started to buck, hesitate and finally died 10 blocks from my driveway. At that point it absolutely refused to restart, it just sat there cranking, with an occasional half pop of ignition. curse
So, I called home and my wife brought my Silverado over with a tow strap and chain and we flat towed the van back to the driveway. At which time it had cooled down again and it restarted instantly. I let it sit and idle until at full operating temp. But it in gear and stomped on it, instant stall.

- I completed the compression test all cylinders fell between 150-170 psi, the service manual says everything within 25psi of each other and nothing below 100psi is OK, so that checks out.

- Random thing, but I could swear I was hearing a slight valve tick from the passenger side valve cover. So I maybe should pull that cover and do a quick check there.

Anyway, I'll dig into some more things eventually. Given when it first broke down I could not even get it to start and idle stably I am technically making progress. laugh Starting and running fine when cold and dying when warm sure makes me inclined to suspect the coolant temp, or throttle body temp sensor. Even though I believe I tested both those a month ago when I was last wrenching. crazy The computer clearly isn't getting good signals from something when the engine is hot.

Cheers,

- Rich


1991 Dodge B250 Zephyr Conversion. "Bi-frost" 74,000-ish original miles and counting.
- 318TBI; has headwork and other bolt on upgrades. Runs strong and smooth!
- Powertrax locker
- 3" aluminum radiator.
- Rear air springs, rebuilt suspension, steering stabilizer.
Re: 1991 TBI; verifying timing advance with RPM
RichardB #783627 October 11th 2021 8:11 pm
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Another quick update. Yesterday I replaced the coolant temp sensor, then let the van idle in the driveway until it got to full operating temp. Then turned it off and successfully restarted it while engine was hot several times. It also moved back and forth in the driveway several times without issue. So another problem solved.
I did not try to drive on the street, wife was not home, so I didn't have a rescue party if the need arose. Next up is to get my wife or my brother to go for a (hopefully) long test drive with me. And by with me, I mean behind me while driving my truck, so that I can again flat tow the van back to my house if needed. And by long, I mean in a lot of circles within a mile or 2 of my house. Hahahaha! What an adventure this is, hopefully soon I can get back to driving and feeling like I can trust this van.

- Rich


1991 Dodge B250 Zephyr Conversion. "Bi-frost" 74,000-ish original miles and counting.
- 318TBI; has headwork and other bolt on upgrades. Runs strong and smooth!
- Powertrax locker
- 3" aluminum radiator.
- Rear air springs, rebuilt suspension, steering stabilizer.
Re: 1991 TBI; verifying timing advance with RPM
RichardB #783721 October 14th 2021 1:46 pm
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pooh-bah
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I'm glad you're making progress! You have plenty of company with the qualification test drives. I change so many things that I go through that process pretty much every time I drive my van!


-It's been such a LONG TIME... BlueShift>> 1981 Dodge Ram B250 Custom Sportsman Maxi Van


[Linked Image]

It's what you learn after you know it all, that counts...

Are you living to work, or working to live?

[Linked Image]

Learning from my own mistakes is good, learning from yours would be much better! [Linked Image]
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